with the same degree of velocity, nearly, that the waIf, therefore, the float-board of a

ter itself, moves. mill wheel that is moveable upon its center, be laid across a river, so as to prevent the water from passing, it will operate as a dam, till the water behind fhall rise to such a height, as, by its prefsure upon the upper side of the float-board, it shall overcome the whole resistance made by the machinery. Whereever this happens, the float-board will be forced to give way and suffer the water to pafs; the succeeding float-board will be made to yield in its turn; and so on, till a rotatory, motion be given to the wheel, that must continue as long as the water fhall continue to flow with the same degree of force.

The principle on which machinery might be turned in these circumstances, is so excefsively clear, that many persons will be surprised it never has been carried into practice in this country; bat when we advert that the power of water, where the fall is considerable, is so much greater than where its motion is lefs rapid, we will not be surprised that mankind fhould have first thought of constructing machinery only where a considerable fall could be obtained; and, when these mills came to be generally used, and the mode of managing water in these circumstances familiar to every body, it would occur at the first glance, that a large body of water, moving slowly, could not be managed with ease in the same way; and of course little attention to sluggish streams, as a moving power, would be given. The difficulties which would thus present themselves, on a superficial view of the subject, might thus appear to be insurmountable, when

they were in fact so easily to be obviated, as scarcely to deserve the name of obstructions at all; as will, I trust, appear from what follows.

The principal reasons why no attempts have been made to construct mills on this plan, are the following:

1st, Were a mill to be placed upon the main body of the stream, or river, there could be no way of guarding against the effects of inundations, by means of sluices, as at present, which turn off as much of the water into another channel, as fhall be at any time superfluous; nor could the flow of the water towards the wheel be entirely prevented when the machinery is meant to be stopped.

To obviate both these difficulties, it would only, however, be required to raise the supports on which the gudgeons of the wheel rest at either end, to such a height as to overtop the wheel; and to make these gudgeons be received into an eye, fixed in a piece of wood, that admitted of being raised upwards at pleasure, in grooves provided in the cheeks for that pur

pose. From each of these boxes let a chain be carried upwards, and passed over a round axle, placed at a sufficient height above the wheel; on one end of which let there be fixed a wheel with spokes, like the wheel of a crane, by means of which, the water wheel might be raised entirely out of the water, whenever it should be wanted to stop the mill, either on account of a flood or otherwise

* As I do not mean here to explain particulars, but merely to develope principles, it is unnecessary to trouble the reader with a detail of the mode in which this might be effected, which could not be rende

2d, A second inconvenience would arise from the increased rapidity of the current during land floods, which would, on these occasions, augment its power so much, as to make the wheel go with an inconvenient degree of velocity; while the wheel would at the same time, by interrupting the current, raise the water behind it to an inconvenient height.

The last of these evils would be entirely removed, by lifting the wheel so far up, by the forementioned contrivance, as to allow the water to pass free below it. By the same means, its power upon the wheel could be moderated, by letting only a small part of the float-board dip into the water*. If, however, this contrivance alone fhould not be found to answer the purpose altogether, many other contrivan-ces, simple enough, might easily be adopted to moderate the rapidity of the current at this place, which it is unnecessary here to enumerate.

Were mills on this principle erected on all the streams that easily admit of it in Britain, machinery, turned by water, might be introduced into many parts of the country, that have been hitherto deemed incapable of any thing of that sort. In rivers that flow through countries which are comparatively flat, this species of mills would answer better than in the rivers that flow through mountainous countries; because these rivers are not so subject to sudden red intelligible to ordinary readers, without many figures. To those who e acquainted with mechanics the above hints will be perfectly sufficient.

It is scarcely necefsary to observe, that this elevation could occasion no derangement to the machinery of the mill, provided an upright spoked trundle of sufficient length, were employed for catching the teeth of the inner wheel.

and violent floods as a mountain stream; and consequently the machinery could be regulated with lefs trouble. In mountainous countries, however, there is less necessity for adopting this contrivance, as falls of water can there be commanded; but even in hilly countries, the streams that ifsue from lakes, of a large size, are peculiarly proper for this purpose, as being lefs liable to sudden inundations than other streams. The Leven, from loch Lomond to Dumbarton, the Awe, from loch Awe in Argyleshire to loch Etive, the Lochy, and the Nefs in Invernessfhire, are large rivers of this kind, on which an infinite number of mills might be erected. On such large streams as these, one wheel might always serve' two mills; one on each side the river. A number of lefser streams are to be found in every part of the country, on which mills of this kind might be erected, on a scale more suited to the general ideas entertained on that subject at present, than these would be; for till enterprises of this sort fhall become more familiar than they now are, those first mentioned would appear too gigantic undertakings for man to atchieve. How long will it be before man fhall come to know the full extent of human powers!

Upon this principle, water, as a moving power, might be commanded in many parts of Scotland, to such an extent, as, comparatively speaking, might be called infinite; and pofsefsing advantages for turning machinery, that cannot be commanded to an equal degree, perhaps, in any other part of the world. But as mankind are apt to be startled, when things that they have been accustomed to look upon as impofsible, are proposed, I fhall not for the present advance

farther in this line of disquisition, reserving what farther might be said on this subject till another occasion.

HINTS ON THE GENERATION OF THE VIPER,

The following extracts respecting the natural history of the viper, have been transmitted to the Editor by a correspondent to whom he lies under very particular obligations for this and many former favours.

Extracts from Mr White's natural bistory of Selborne, published 1789, relative to the viper.

To Mr Pennant.

"PROVIDENCE has been so indulgent to us, as te allow of but one venomous reptile of the serpent kind in these kingdoms, and that is the viper. As you propose the good of mankind, to be an object of your publications, you will not omit to mention common salad oil, as a sovereign remedy against the bite of a viper. As to the blind worm, (anguis fragilis, so called, because it snaps asunder with a small blow,) I have found upon examination that it is perfectly innocuous. A neighbouring yoeman (to whom I am indebted for some good hints,) killed and opened a female viper about the 27th of May: he found her filled with a chain of eleven eggs, about the size of those of a blackbird; but none of them were advanced so far towards a state of maturity, as to contain any rudiment of young. Though they are oviparous, they are viviparous also, hatching their young within their bellies, and then bringing them forth: whereas snakes lay chains of eggs every summer in my