J'ai un homme qui me rendra service,

J'ai besoin d'un homme qui me rende service,

I have a man who will oblige me.

me.

4. A verb preceded by a superlative, relative, or by the words le seul, le premier, le dernier, is put in the subjunctive [§ 127].

Voilà le seul chapeau que j'aie, Voilà le meilleur homme que je connaisse.

That is the only hat I have. There is the best man I know.

1. Pensez-vous que ce drap dure longtemps? 2. Je crois qu'il durera bien, car il est fort. 3. Croyez-vous que notre portier tarde à rentrer? 4. Je crois qu'il ne tardera pas. 5. Désirez-vous que nous restions debout? 6. Je désire, au contraire, que vous vous asseyiez. 7. Croyez-vous que ces étudiants puissent apprendre cinq pages par cœur en deux heures? 8. Je crois que c'est impossible. 9. Espérez-vous que notre ami arrive de bonne heure? 10. J'espère qu'il arrivera bientôt. 11. Quelle sorte de carafe vous faut-il? 12. Il m'en faut une qui contienne un litre. 13. J'en ai une de cristal qui contient deux litres. 14. Pensez-vous que ce négociant s'enrichisse à vos dépens? 15. Je sais qu'il s'enrichit aux dépens d'autrui. 16. Quel parasol pensez-vous me prêter? 17. Je pense vous prêter le meilleur que j'aie. 18. Le tanneur réussira-t-il à gagner sa vie? 19. Je ne crois pas qu'il y réussisse. 20. Pensez-vous que cet argent suffise à votre père ? 21. Je crois qu'il lui suffira. 22. Croyez-vous que ces messieurs comptent sur moi? 23. Je sais qu'ils comptent sur vous. 24. Pensez-vous que le concert ait lieu aujourd'hui ? 25. Je crois qu'il n'aura pas lieu.

EXERCISE 142.

1. Do you believe that the concert has taken place? 2. I believe that it has taken place. 3. Do you believe that your sister's dress will wear well? 4. I think that it will wear well, for the silk is very good. 5. Do you believe that our friend will succeed in earning a livelihood? 6. I believe that

he will succeed in it (y), for he is very diligent. 7. Do you think that the tanner grows rich at my expense? 8. I think that he enriches himself at the expense of others. 9. Does the merchant grow rich at my father's expense? 10. He grows rich at your expense. 11. What kind of a house must you have (vous faut-il)? 12. I must have a house which has ten rooms. 13. I have a good house which has twelve rooms. 11. What kind of decanter do you seek? 15. I look for one that holds three litres. 16. I have one which holds two litres, I will lend it to you. 17. What coat will you send me? 18. I will send you the best I have; take care not to stain it. 19. Do you think that the student will learn all that by heart? 20. I do not think that he will learn it. 21. Do you believe (that) he will come? 22. I believe that he will come soon. 23. Do you think that your father depends upon me? 24. I

4. This tense may be formed from the past definite [Sect. L.) by changing, for the first conjugation, the final i of the first person singular of the past definite into sse, sses, etc., and by adding se, ses, etc., to the same person in the other three conjugations. This rule has no exceptions.

J'allai, j'allasse; je finis, je finisse. I went, I might go; I finished, I might finish.

5. All the observations made in Sect. LI., on the changes of the stem of the irregular verbs, in the past definite, apply equally to the imperfect of the subjunctive.

6. The pluperfect of the subjunctive is formed from the imperfect of the same mode of one of the auxiliaries avoir, être, and the past participle of the verb :Que j'eusse fini; que je fusse venu.

That I might have finished; that I might have come.

7. All the rules given on the use of the subjunctive in the three preceding sections, apply, of course, to the imperfect and pluperfect of the mode.

8. In the same manner as the present or future of the indicative of the first part of a proposition governs, under the above-mentioned rules, the verb of the second part, in the present or past of the subjunctive; so the imperfect and other past tenses of the indicative, and the two conditionals, govern the verb in the second part of the proposition, in the imperfect or pluperfect of the subjunctive. Ne fallait-il pas que je lui parlasse? Il faudrait que je lui donnasse ce livre,

RÉSUMÉ OF

Voudriez-vous que je donnasse un coup de bâton à cet enfant ? Je voudrais que vous tirassiez un coup de fusil sur cet oiseau.

Exigeriez-vous que nous revinissi

ons de bonne heure ?

Que voudriez-vous que ces hommes

fissent ?

Que vouliez-vous que je fisse ?
Il faudrait que j'eusse mon argent?

Je ne voulais pas que vous mourussiez de froid.

Elle craignait que vous ne mourussiez de misère et de faim. Voudriez-vous que je jetasse un coup-d'œil sur ces papiers?

Was it not necessary that I should speak to him?

It would be necessary for me to give him that book. EXAMPLES.

Would you wish me to give that child a blow with a stick?

I would wish you to fire your gun at that bird.

Would you require us to return early?

What would you wish those men to

do?

What did you wish me to do?
It would be necessary for me to have

my money?

I did not wish you to die with cold.

She feared lest you might die with want and hunger. Would you wish me to cast a glance upon these papers?

1. Voudriez-vous que j'achetasse un habit à demi-usé? 2. Je voudrais que vous en achetassiez un neuf. 3. Voulait-on que ce soldat malade se rendît à son poste? 4. On voulait qu'il se rendît à son régiment. 5. Faudrait-il que je demeurasse au bord de la mer? 6. Il faudrait pour le rétablissement de votre santé que vous vous restassiez en Suisse. 7. Ne pensez-vous pas que cet enfant ressemble à sa mère? 8. Je ne pense pas qu'il lui ressemble. 9. A qui ressemble-t-il? 10. Il ressemble à sa sœur aînée. 11. Consentiriez-vous que votre fille épousât cet ivrogne? 12. Voudriez-vous que nous mourussions de misère? 13. Je craignais que ces dames ne mourussent [§ 127 (8), Sect. LXXI., 9] de froid. 14. Ne voulez-vous pas tirer sur ce lièvre ? 15. Je tirerais sur cette bécasse si mon fusil était chargé. 16. Combien de coups de fusil voudriez-vous que je tirasse? 17. Si vous aviez de la poudre, je voudrais que vous tirassiez sur cette perdrix. 18. Voulez-vous que je jette un coup d'œil sur cette lettre? 19. Je voudrais que vous la lussiez. 20. Que voudriez-vous que je fisse? 21. Je voudrais que vous fissiez attention à vos études. 22. Faudrait-il que je sortisse ? 23. Il faudrait que vous restassiez à la maison. 24. Que voudriez-vous que je fisse à ce cheval? 25. Je voudrais que vous lui donnassiez des coups de fouet.

EXERCISE 144.

1. What would you have me do? 2. I would have you cast a glance upon this letter. 3. Would you wish me to give that dog blows with a stick? 4. I would wish you to give that horse blows with a whip. 5. Would you require us to return at five o'clock? 6. I would require you to return early. 7. Do you think that your brother resembles your father? 8. I do not think he resembles my father. 9. Whom do you think he resembles ? 10. I think he resembles my mother. 11. How many shots have you fired? 12. I have fired five shots at that woodcock. 13. Would you not have me fire at that partridge? 14. I would have you fire at that partridge, if your gun were loaded. 15. Where would it be necessary for me to dwell? 16. It would be necessary for you to dwell on the sea-shore. 17. Would you have me die with hunger? 18. I would not have you die of hunger. 19. Would you have your brother die with cold? 20. I would not have him die with cold or want. 21. What would you have your son do? 22. I would have him learn his lessons. 23. Would you have him learn German? 24. I would have him learn German and Spanish. 25. Have you fired at (sur) that hare? 26. I have not fired at that hare. 27. Would it be necessary for me to go out? 28. It would be necessary for you to go out. 29. Would it be necessary for me to remain here? 30. It would be necessary for you to go to church. 31. What did you wish? 32. I wished you to write to me. 33. Did you wish me to buy a coat half worn out? 34. I wished you to buy a good hat.

KEY TO EXERCISES IN LESSONS IN FRENCH.
EXERCISE 54 (Vol. I., page 276).

1. Notre médecin sait-il le français? 2. Il sait le français, l'anglais et l'allemand. 3. Connaît-il le médecin français? 4. Il le connait trèsbien. 5. Connaissez-vous cette dame? 6. Je ne la connais pas. 7. Est-elle allemande ou suédoise? 8. Elle n'est ni allemande ni

suédoise, elle est russe. 9. Avez-vous l'intention de lui parler? 10. J'ai l'intention de lui parler en anglais. 11. Sait-elle l'anglais ? 12. Elle sait plusieurs langues; elle parle anglais, danois, suédois et hongrois. 13. Monsieur votre frère est-il colonel? 14. Non, Monsieur, il est capitaine. 15. Votre tapissier est-il danois? 16. Il n'est pas

danois, il est suédois. 17. Etes-vous français? 18. Non, Monsieur,

je suis hongrois. 19. Savez-vous le chinois? 20. Je sais le chinois, le russe et le grec moderne. 21. Avez-vous tort d'apprendre les lani pas tort d'apprendre les langues. 23. Connaissez

27. J'aime les livres. 28. Avez-vous envie d'apprendre le russe? 29. Je n'ai pas envie d'apprendre le russe. 30. N'avez-vous pas le temps? 31. Je n'ai guère de temps. 32. Qu'apprenez-vous? 33. Nous apprenons le latin, le grec, le français et l'allemand. 34. N'apprenezvous pas l'espagnol? 35. Nous ne l'apprenons pas. 36. Avez-vous de belles fleurs dans votre jardin? 37. Nous avons de très-belles fleurs; nous aimons beaucoup les fleurs. 38. Les lui donnez-vous? 39. Je vous les donne. 40. Donnez-nous-en. 41. Ne nous en donnez pas. EXERCISE 55 (Vol. I., page 294).

1. Whom do you know? 2. We know the Dutchman of whom you speak to us. 3. What lessons are you learning? 4. We are learning the lessons which you recommend to us. 5. Is what I tell you true! 6. What you tell us is true. 7. Of whom do you speak to us? 8. We speak to you of the Scotchmen who are just arrived. 9. Do you know brother is acquainted is just arrived. 11. What are your sisters doing? who is just arrived? 10. I know that the gentleman with whom your 12. They do almost nothing, they have almost nothing to do. 13. What do you put into your trunk? 14. We put what we have, our clothes and our linen. 15. Do you not put your shoes? 16. We put in the shoes which we want. 17. What do you want? 18. We want what we have. 19. Does that child know what he is doing? 20. He knows what he does and what he says. 21. Will you not tell them of it? 22. With much pleasure. 23. Are you doing what the merchant orders? 24. We do what he tells us. 25. He speaks of that of which you speak. EXERCISE 56 (Vol. I., page 294).

1. Avez-vous ce dont vous avez besoin? 2. Nous avons ce dont nous avons besoin. 3. Le monsieur que vous connaissez est-il ici! 4. La dame dont vous parlez est ici. 5. Vient-elle d'arriver? 6. Elle vient d'arriver. 7. Connaissez-vous ce monsieur? 8. Je connais le monsieur qui parle avec M. votre père. 9. Savez-vous son nom? 10. Je ne sais pas son nom, mais je sais où il demeure. 11. Que faites. vous tous les matins ? 12. Nous ne faisons presque rien, nous n'avons presque rien à faire. 13. Le tailleur fait-il vos habillements? 14. I fait mes habits, ceux de mon frère et ceux de mon cousin. 15. Savezvous ce que vous dites? 16. Je sais ce que je dis et ce que je fais. 17. Connaissez-vous l'Écossais dont parle M. votre frère? 18. Je le connais bien. 19. Que met-il dans son coffre ? 20. Il y met ses habil

lements. 21. Ce que vous dites est-il vrai ? 22. Ce que je dis est

vrai. 23. Comprenez-vous ce que je vous dis? 24. Je comprends tout ce que vous dites. 25. De qui M. votre frère parle-t-il? 26. I parle du monsieur dont la sœur est ici. 27. M. votre frère a-t-il tort de faire ce qu'il fait ? 28. Il ne peut avoir tort de le faire. 29. Que faites-vous ? 30. Je fais ce que vous faites. 31. Où mettez-vous mes livres ? 32. Dans le coffre de M. votre frère. 33. M. votre frère est il ici? 34. Il n'est pas ici. 35. Il est chez mon frère ou chez mon père.

HYDROSTATICS.-I.

OBJECTS OF THE SCIENCE-PRINCIPLE OF EQUALITY OF PRESSURE-HYDROSTATIC PRESS.

THE branch of Natural Philosophy the study of which we are cerned in examining the conditions of equilibrium in liquids, the now about to commence is called Hydrostatics, and it is conpressures they exert, and their motions; just as Mechanics was concerned with solid bodies.

All matter exists in one of three states-the solid, the liquid, or the gaseous; and the sciences of Mechanics, Hydrostatics, and Pneumatics treat respectively of its motions and the forces which act upon it in these three states. We must not, however. imagine that a body can exist in only one of these conditions, for many assume at different times all three. To take the simplest illustration, water is best known to us in a liquid state, that being the one in which we most commonly meet with it; but if a certain amount of heat be taken away from it, it will become changed into a solid, which, though to distinguish it, we call ice, is not a new substance, but merely the water different state. So also an increase of heat will change the water into an invisible gas or vapour which we call steam. In this case heat is the agent which produces these changes of state, and it does so by driving the ultimate particles of the substance farther apart from each other. Many of the metals, heat, and hence can be melted and cast into moulds of any as is well known, assume the liquid state under the influence of heat, as may be done in the electric lamp, they, too, will become desired shape. If they be exposed to a much higher degree of

converted into vapour.

le connais pas.

26. Aimez-vous les livres ?

៖

attraction for each other, that is, cling closely together, and resist any effort to separate them. Many of the metals can be drawn out into fine wires, and yet will sustain considerable weight before the attraction or cohesion, as it is termed, is overcome. If we take two lead bullets, and scrape a portion of the surface of each so as to render them even, and then by pressing them firmly together, drive out the air, this cohesion will cause them to cling to each other so tightly as to require a considerable degree of force to separate them. Another property of solids which results greatly from this, is the amount of friction with which their ultimate particles move over one another. In some solids this is so great that no moderate degree of force will suffice to move them or to alter the form of the mass. In this respect there is, however, a great difference between solids, for they merge so gradually into liquids that it is difficult to draw a well-defined line separating them.

In liquids both these properties are present in a much smaller degree. The cohesion of the particles is so much less that scarcely any force is required to separate a mass of liquid into portions; in fact, it falls apart from its own weight, unless it be put into some vessel capable of containing it, and it immediately assumes the shape of such a vessel. The same, however, might be said of a heap of fine powder: how then does this differ from a liquid? The difference consists, first, in the fact that there is a large amount of friction between the atoms of powder, so that if placed in a heap they do not spread themselves out as particles of liquid would; and next, in the ultimate atoms of the liquid being so minute as even under the most powerful microscope to be invisible, while those of the powder have a definite size. The property the particles have of moving over one another with scarcely any friction is one of very great importance, and accounts for several of the phenomena we shall meet with.

If we now look at the case of a gas we shall find that not only is there no cohesion between the particles, but they repel one another, and, unless confined, will fly apart as far as possible. If a cubic inch of any gas be placed in a large box, it will immediately fill it and become equally distributed in every part. There is also this further difference between liquids and gases, that whereas a gas may be compressed almost indefinitely, regaining its former bulk on the pressure being removed, a liquid is for all practical purposes incompressible.

It was for a long time believed to be absolutely so, but it has since been found that a pressure equal to that of the atmosphere, or 15 pounds per square inch, will cause a compression in water to the extent of 40 or 50 millionths of its bulk. The simplest way of ascertaining this is to procure a cylinder closed at one end, and having a piston fitting very tightly into it. This is filled with water, and a spring ring placed just under the piston, so that if it be driven in at all, the ring will remain at the part of the cylinder which it reached, and thus show the extent of the compression. The apparatus thus arranged is fixed to a heavy weight, and by means of a cord lowered to a known depth in the sea. The pressure, as will be seen. increases with the depth, and the position of the ring will indicate the extent of the compression.

Having thus cleared our way, we can enter more directly on the science itself. It is usually divided into two brancheshydrostatics proper and hydrodynamics; the former treating of the equilibrium of liquids and the pressures they produce, while the latter has to do with their motions. The term hydraulics, derived from two Greek words meaning "water and "a pipe," is sometimes used instead of hydrodynamics, but it is more strictly applied to the raising of water by means of pipes; we shall, however, use it in its more extended meaning. Water is by far the most common of all liquids, and hence will be taken as a type. In its physical properties, however, it differs little from other liquids, and what is said of it may, with the necessary modifications, be applied to liquids generally.

We found in Mechanics that though the lever and other mechanical powers possessed weight, we could understand their principles better by neglecting it at first; just so here it is easier to omit at first all notice of the weight of the liquid.

The fundamental principle of hydrostatics is that of the equality of pressure, or, as it is sometimes called, after the philosopher who first stated it, Pascal's law. It is as follows: -If any pressure be exerted on any part of a liquid, that pres

sure is transmitted equally and with equal force in all directions. A little explanation will make this clear. If we have a solid eylinder made to fit exactly and move without friction in a tube, and we press with any force against one end of it in a direction parallel to its length, the pressure will be transmitted undiminished to the other end, and will there act against any obstacle just as if the cylinder were not interposed; no pressure will, however, be exerted against the side of the tube. If now the cylinder be removed and the tube filled with water, a piston being made to fit each end of it, any pressure exerted on one end will, as before, be transmitted to the other, but a similar pressure will also be exerted against every part of the inner surface of the tube. If the surface of the piston have an area of one square inch, and a pressure of 10 pounds be exerted on it, every square inch of surface in the cylinder will sustain a similar pressure; and if we insert into any part of it a tube with a piston one square inch in area, this piston will be forced out with a pressure of 10 pounds. If the tube be bent or twisted in any direction, the pressure is still transmitted exactly as if it were perfectly straight. This property of liquids follows from the fact of their particles moving without friction, and is of great practical importance. In Mechanics, even with the best and most flexible ropes and chains, there is always a great loss from friction and rigidity, but by means of a liquid a pressure can be transmitted in any direction without sensible loss. Similarly, if we have a closed vessel with several equal openings in it, in each of which a piston of one inch diameter works, a pressure of 10 pounds on one will cause a similar pressure on each of the others. If now another piston be fitted to the vessel, 10 inches in diameter, a pressure of 10 pounds will be exerted on every portion of its surface equal in area to the smaller piston. Now the areas of circles are proportional to the squares of their diameters; the area of the larger piston is therefore 100 times that of the small one, and the total pressure on it is therefore 100 x 10, or 1,000 pounds. We have thus what we may consider as another mechanical power, a gain being effected by the use of it as there was by the lever. The principle of virtual velocities holds good here as well as in the powers we previously considered; for if the small piston be forced in 1 inch, it is clear that the other will only be moved to the extent of th of an inch, and thus, though 100 times the pressure is exerted, it is only throughth part of the space. A simple experiment can easily be tried to show that this pressure is transmitted upwards as well as in other directions. Procure a tube of large diameter, and grind one end of it flat, so that it can be closed by a disc of glass fitting closely against it. Fasten a piece of string to the middle of this disc, and pass the end up through the tube, so that by holding the string it may be kept in its place. If the whole be now lowered into a vessel of water, the upward pressure will keep the disc in its place without the string being held; but on the tube being gradually raised till the end comes nearly to the surface, the pressure will diminish until it will be unable any longer to sustain the disc, which will then fall to the bottom.

This principle of equality of pressure leads to many strange and important results. The apparatus known as the hydrostatic bellows, and represented in Fig. 1, is an interesting illustration of it. Two circular boards, A, B, are connected together by flexible sides, so as to form a circular pair of bellows. A long tube opened out at the upper end into the shape of a funnel is made to open into this. The whole arrangement is then partly filled with water, not, however, so much so as to fully expand the bellows. If now several heavy weights be placed upon the upper board, and water be poured down the tube, the weights will be raised. A man may even stand on the board, and, if the tube be long enough, be raised by pouring in a jug of water. Let us suppose that the area of the tube be half a square inch, and that of the board 1 foot, and that a pound of water be poured into the tube. The pressure on the bottom of the tube is, of course, 1 pound; and a similar pressure is exerted on every half a square inch in the inner surface of the bellows. Now the surface of the board is 144 square inches, the pressure produced on it by the pound of water is therefore 288 pounds. On account of the great apparent gain this is sometimes called the Hydrostatic Paradox, but on consideration it is seen to be no more paradoxical than the gain effected by any other mechanical power; as, always, what is gained in power is lost in time.

In a similar way, if a cask be filled with liquid, and a long tube be inserted tightly into the top and filled also, the weight of water in this tube, though trifling in itself, will exert such a pressure as to burst open the cask. If the diameter of the pipe be rather over one-third of an inch, its area will be about one-tenth of a square ineh, and 2 pounds of water poured into it will fill it to a height of about 46 feet. This will cause a pressure of 20 pounds per square inch; and as the surface may have an area of about 2,000 square inches, the total pressure produced by the 2 pounds of water will be about 40,000 pounds, a pressure sufficient to burst almost any cask. For this reason, when the parts of a town lie at different levels, and the water is supplied from a reservoir situated in an elevated part, the pipes have to be made very strong, as they have to sustain the pressure produced by a column of water as high as the most elevated part is above them.

Perhaps the most important application of this principle is seen in the hydrostatic press, an instrument which is largely used in the manufactures where a very powerful pressure is required. The general principle on which it acts is simply this: by means of a piston or plunger, of small diameter, water is forced into a cylinder in which a large piston works. The latter is forced out with a pressure as much greater than that exerted on the plunger, as its area is larger. If the large piston have 20 times the diameter of the small one its area will be 400 times as large, and therefore a pressure of 1 pound will exert a force of 400.

Fig. 3.

The advantage gained by the machine depends upon two things: first, the comparative sizes of the pistons; and second, the proportion between the distances A B and H B. Suppose, for example, that the diameter of K is 15 inches, and that of D half an inch; also that the length of A B is 3 feet and B H 2 inches, what power will a man pressing at A with a force of 50 pounds exert? Since the diameters are in the proportion of 1 to 30, their areas are in the proportion of 1 to 900, and therefore whatever pressure is exerted on D, K sustains 900 times the amount. But AB is a simple lever of the second kind, and w the gain produced by its use is 3, or 18. The pressure of 50 pounds at A produces, therefore, one of 900 on D; and thus the total force with which the books at M are compressed is 900 x 900 810,000 pounds, or upwards of 360 tons. As we can increase the length of the lever or diminish the diameter of the small piston greatly, the only practical limit to the power of the machine is the strength of the large cylinder. An additional tap, not shown in the figure, is arranged so that the water can be allowed to escape from the cylinder when it is desired to remove the pressure.

The annexed figure (Fig. 2) will give a good idea of the general construction of the machine. It varies, however, greatly in shape and minor details according to the power required or the special purpose to which it is to be applied. A B represents a lever hinged at B to an upright, and working a solid plunger, D, which is so arranged as only to move in a vertical direction. This plunger, which is of small dimensions, works watertight in a small cylinder, c. When it is raised by means of the handle, A, water rises from the reservoir, G, into the cylinder through the valve, E, which then closes and prevents the water passing back again. The piston is now forced down, and drives the water through the pipe into the large cylinder, L, and thus communicates the pressure it receives from D. A second valve is placed at F, so as to keep the water from flowing back when D is raised again. The making of the large cylinder, L, requires the greatest care, on account of the immense pressure it has to withstand. When the Great Eastern steamer had to be launched, some presses were used in which the cylinder was made of iron 7 or 8 inches thick, and yet they were split open by the immense strain exerted on them. The piston, K, is made of as large dimensions as practicable, as on

This machine is constantly used by manufacturers for many different purposes. Oil is expressed from seeds by it, goods are compressed for packing, and many other operations performed. The most remarkable application of it, however, was in the building of the celebrated tubular bridge over the Menai Straits.

The longest of the tubes of which this is constructed is 472 feet long, and the inside dimensions are about 14 feet wide, and 25 feet high. Each weighs about 1,200 tons: they were completely finished, and then conveyed on pontoons to the spot, and raised into their place by an hydraulic press, which was the largest ever constructed. Its cylinder was 22 inches internal diameter, 10 inches thick, and 9 feet long, and weighed upwards of 15 tons. The cross pieces to which the lifting chains were attached weighed 13 tons.

The lifting force this press was capable of exerting was esti mated at over 2,000 tons. It was erected at the top of the towers, and chains were fastened round the tube and attached

this mainly depends the power of the machine. It works watertight through a collar at I, and it was here that the greatest practical difficulty was found, for the water oozed through, and thus the power of the machine was greatly diminished. The difficulty was at last met by making a collar of stout leather, and so that its section was of the shape of the letter U inverted. This ring is placed in a groove prepared for it, and it is easily seen that when the water presses on it, it tends to open the bend, and thus causes the collar to press more firmly against the piston. The books or other articles to be compressed are laid upon the flattened top of the piston, and pressed against a mework constructed for the purpose.

r

to the cross head of the ram. At each lift the tube was raised about 6 feet, the space underneath was then filled in with masonry, and the chain shortened for another lift; and in this way each tube was raised to its elevation of 101 feet above high-water mark.

As valves are of frequent employment in hydraulic machines, it will be well just to give an explanation of their construc tion. The object to be attained by their use is to allow water to flow freely along a pipe in one direction, but to prevent it passing in the other. The simplest means of accomplishing this is by a common clack valve (Fig. 3, a). This consists of a plate of metal, hinged at one edge, and closing over the opening. A piece of leather larger than the plate is usually placed over the upper surface, and the weight of the water presses it against the other surface, thus closing the valve more tightly. Another kind of valve is shown at b, and consists of a ball which rises by the pressure of the water, but falls by its own weight, and closes the pipe when the water tries to pass back. The other variety in common use is the spindle valve shown at c. A conical piece of metal is made to fit into a setting, and confined by a guide rod so as to rise vertically. The best inclination for the sides of the valve is an angle of about 45°, as then there is little danger of its becoming stuck in its setting, and on the other hand it does not waste much room.