in addition to this, piles were driven into the water obliquely, at the lower side of the bridge, and these, serving as buttresses and being connected with every portion of the work, sustained the force of the stream, and there were others also above the bridge, at a moderate distance; that if trunks of trees or vessels were floated down the river by the barbarians for the purpose of destroying the work, the violence of such things might be diminished by these defenses, and might not injure the bridge.' Cæsar's bridge was practically a repetition of the simple bridges that we see on country roads over the brooks, with the exception that he built a series of them and put in piers to support the ends of each section. The strength of the bridge depended wholly upon the stiffness of the timbers. If one could imagine a series of brooks flowing side by side with a bridge over each, the result would be the same as Cæsar's bridge, with the exception that the narrow banks between the parallel brooks would be equivalent to the posts that Cæsar put at the different sections. 3 5 7 2 CBDAYS The next step in strengthening a bridge so that it shall not depend entirely upon the stiffness of the timber, is to put in on each side a truss shaped like a very broad A. This form of truss is used where the span is so great that the timbers laid from bank to bank need additional support. The vertical rod running from the timber underneath through the apex of the letter A holds up the center of the beam and prevents it from sagging. These sketches are referred to by numbers in the accompanying article. The lower ends of the A are kept from spreading by being bolted through the horizontal beam. In our diagrams are shown these various types of bridges, as follows: No. I may be used if the timbers are too short to reach across from one pier to another, and more than one length is needed. Where they are joined the rods beneath carry the weight, and hold up the floor timbers by supporting the upright post. No. 2 is the same, only, instead of rods and beams, it is made entirely of planks or iron girders bolted together, and the A may be under or over the bridge. In No. 3 there are two supporting posts (instead of a single one) united by a horizontal beam. In the arch bridge in No. 4, the principle is about the same as in the truss, except that in the arch it is the abutments that keep the ends from spreading. If the arc of the arch bridge is less than a half-circle, the ends will tend to spread. If the arc is a complete halfcircle, the ends will not spread, but will rest with the weight pressing vertically on the piers. The arrows in diagrams No. 4 and No. 5 show the direction of the weight, and it will be readily seen that in No. 4 this weight tends to push the SKETCHES TO ILLUSTRATE VARIOUS FORMS AND METHODS ends apart; but such spreading is prevented by the abutments. In No. 5 the weight on the ends is directly downward. This bridge No. 6 shows the principle of the rolling liftbridge, a form of drawbridge. The solid part is heavy, so as to balance the truss part, and to enable the bridge to be easily lifted out of the way of passing boats. One of these sections is on each side of the stream, and, when closed, the ends come together at the center. operates in a way similar to that of the weighted aluminium pepper-shaker figured in No. 7. The lower part of the pepper-box is a solid weight, which keeps the box right end up, and the lower part of the bridge No. 6 is a heavy weight that assists in raising the rest of the bridge. In southern Africa there is a very high bridge over the Zambesi River. The workmen first fired a rocket over the river. To this rocket was attached a cord. The cord pulled a wire, and then the wire pulled a small cable across the river. On this cable was suspended a truck which took over the main cable of the bridge, and so, piece by piece, the bridge was gradually put together. A REMARKABLE PHOTOGRAPH OF LIGHTNING NOVEMBER II, 1911, was, in northern Michigan, a typical autumn day with short showers at intervals. On my way home, at about five o'clock in that I had got a picture, perhaps more than I wanted. When I developed the plate, I had the accompanying photograph, at which I often look and always with joy. The next day, as I was bragging a little, one of my friends said that that lightning flash struck his clothes-post, cut it off close to the ground, jumped across to his neighbor's house, tipped over the sewing-machine, and tore the linoleum from the kitchen floor. Such a flash as that deserved to have its picture taken, and here it is. A. B. COVEY. STRIKING CONTRAST IN METHODS OF FEEDING EVERY observer of the giraffe must see that nature has intended that the animal's food should be found at a considerable distance above the ground. The long legs and the very long neck suggest that, and observation proves it, since the giraffe, in its native haunts, gathers its food of leaves from the branches of the trees. But it does not obtain all its nourishment in this way; it sometimes feeds from the ground. Of course its long legs and long neck are then hindrances rather than helps, so it has adopted an original device to overcome this trouble. This is to spread its front legs far apart, which, of course, lowers the body, and brings the head nearer the ground. The accompanying photograph is remarkable because it shows both methods of feeding, as exemplified by the two Nubian giraffes, "Romeo" THE REMARKABLE PHOTOGRAPH OF LIGHTNING. the afternoon, the rain began to fall in torrents, and the lightning flashes were so vivid and, in some particulars, so interesting to a photographer, that I resolved to take a picture of a flash, as I had long desired to do. I set the camera on my porch, directed it toward that part of the landscape that would make a good foreground, opened the slide, and went to supper. But just before I sat down at the table, I looked out of the window and was met by an awful flash. I felt METHODS OF HIGH AND LOW FEEDING. and "Juliet," in the New York Zoological Park, where the giraffe feeding from the tree is reaching upward to a distance of twenty feet. HOW AN ELEPHANT'S TEETH GROW THE elephant has no cutting teeth like most animals, but only a series of molars. These molars, or grinders, as they wear away, gradually move forward in the jaw, and the remnant of the tooth, when the surface is completely destroyed, is cast out in front. The same molar can thus be replaced as many as eight times. The tusks, which are only enormously elongated teeth, can be renewed only once. This wearing process and the ejection of the stump of the tooth go on very slowly during the life of the elephant. Only one or two teeth at a time are in use, or in view, in each jaw. There are always other teeth waiting to pass forward and begin their work, although there is a limit to this succession, for, when the last has come into THE PRICKLY-PEAR GROWING UP IN A TREE. its nourishment from the air, and not from the tree. The seed was probably carried to the place by a bird, and the germination and development of the plant to its present size show its wonderful vitality, especially when we consider the arid climate in which it grows. LESLIE L. LONG. A HORNETS' NEST IN AN OLD PHAETON THE accompanying photograph, showing a hornets' nest built in the cover of an old phaëton, was sent to ST. NICHOLAS by W. T. Ness, Cambridge, Massachusetts. The shed in which the phaëton stood was closed for the summer, and A DIAGRAM OF THE MOUTH AND JAWS OF AN ELEPHANT-SIDE VIEW. A and B show the teeth that have been pushed forward to the front of the mouth and are in use. When they are worn out, their place is taken by D and C, which work forward to supply the vacancy. The undeveloped teeth at E and F come forward to take the place of D and C, and later, as they grow larger and stronger, of A and B. use and been worn down, the elephant can no longer chew his food, and must die of starvation, if he has not already succumbed to old age. E. G. LUTZ. PRICKLY-PEAR IN A TREE! THIS picture shows a prickly-pear, Opuntia lindheimeri, growing on a mesquit-tree, Prosopis juliflora, in the crevice of a limb about ten feet from the ground. It is three to four years old, and has received THE HORNETS' NEST HANGING FROM THE TOP OF A PHAËTON. the hornets carried on their nest-building without interruption. Surely this is a novel place for a hornets' nest, and it would have made things decidedly unpleasant if the owners had attempted to take a ride. While we keep no record of such occurrences, we do know that it is not at all unusual for objects like polliwogs, small fish, etc., to be sucked up, as it were, from water surfaces in strong currents. These are carried for distances, depending upon the wind velocity, after they have reached their highest altitude, and descend as soon as the force of gravity is sufficient to overcome the sustaining force of the wind. At times, such small objects are carried many miles before again coming to earth. I am afraid that the suggestion of the little girl as to the hatching out of the polliwog eggs while in the air will hardly hold.-H. E. WILLIAMS, Acting Chief U. S. Weather Bureau. At first thought, the suggestion that polliwogs had hatched in the air does seem ludicrous. And yet let us give it a little careful thought and see if it is impossible. Hatching we understand to mean the breaking of the living form out of its egg case. Frogs' eggs are so fragile that even a jarring of the masses may break the cases and let loose many lively tadpoles. If it is admitted that the water as it is sucked up takes up small TADPOLES READY TO BREAK OUT OF THE FRAGILE EGGS. objects with it, then why not frogs' eggs? It seems not impossible that the tadpoles might be hatched out by the shaking caused by strong currents of wind, or by jostling against other "small me about the flowers and cacti; and yesterday we saw the funniest little toad, all covered with thorns. He was squatted right down in an "ant road." You know the ants make little paths to different places; well, this funny "horned toad," as my uncle says it is called, was there, eating the ants. He just opened his mouth and they would run right in, and he just ate and ate as if they were as good as ice-cream. Then Uncle picked that toad up in his hands, and showed me that it would n't hurt you at all. And then he put it in his pocket. We found another one, and when we got back home, he took their pictures. Don't you think them funny little toads? MIRIAM GRACE DUNGAN. P.S. We turned the toads out the next morning so they could eat more ants. DO SNAKES TRAVEL AT NIGHT? SALAMANCA, N. Y. DEAR ST. NICHOLAS: Will you please tell me through "Nature and Science" if snakes come out or travel at night, especially rattlesnakes. Your friend and reader, Many of the poisonous species of snakes travel at night. The greater number of our harmless snakes are day prowlers. The rattlesnake and copperhead snake are largely nocturnal in hunting their food, although they delight to bask in the sunlight. As an indication of their nocturnal habits, it is interesting to note that the pupil of the eye of these snakes is elliptical like that of a cat. RAYMOND DITMARS. SOME BIRDS WHO LIKED A SHOWER-BATH LOUISVILLE, KY. DEAR ST. NICHOLAS: I wish to tell you about a pretty sight I witnessed while visiting my aunt who resides in Indianapolis. My aunt had rented a house for the summer. It was in the district of the city where there are a great many trees. In the yard was a large variety of beautiful flowers, and I had made it my duty to give these flowers water. On a certain day I was in the garden giving them their daily watering. I had adjusted the hose to a spray, when, from a near-by fence, a robin flew and perched right on a stem which was exposed to the water, where he took a bath and seemed inclined to stay. In a few moments two other birds, of bluish color, also came and took a bath. For about three minutes they stayed under the spray, and if I turned the hose away, they would fly after it. Then they flew up on the fence to take a sun-bath. I then turned the water off and went into the house, where, on questioning my aunt, she informed me that the owner of the house always gave the birds water and crumbs and let them bathe, and so made them very tame. Another time as I went to turn on the water, I noticed a leak where the hose was attached to the pipe, and under that, in the shade, a sparrow was taking a shower-bath, being not in the least frightened when I came up. These incidents impressed me very much, as in my home city, at least in the part where I live, we seldom see any bird but the common sparrow. OLGA ALMA TAFEL (age 15). LOOKING AT THE RISING OR SETTING SUN WASHINGTON, D. C. DEAR ST. NICHOLAS: I have often wondered why we can look at the sun at its rising and setting and not at mid-day. I would be very glad if you would answer my question. Yours sincerely, KATHARINE WARD. The reason why we can look at the sun when it is rising or setting, but not, usually, when it is high in the heavens, is merely because some of its light is absorbed in passing through our air, for the air is not perfectly transparent. Now when the sun is near the ground, its rays come to us almost horizontally, traversing hundreds of miles. of our wet and dusty air before they finally reach our eyes; consequently, a much larger portion of the light is absorbed than when the sun is high up in the sky, and its rays pass through one hundred miles of air only, the upper nine tenths of which is so rare that it absorbs but little of the light. It is an interesting fact that our atmosphere lets red light pass through it more easily than colors that are higher in the spectrum-that is, nearer to a blue color. So that when the air is unusually dusty, and hence not so transparent as usual, very much of the blue part of the sun's light is wholly cut off, and its disk then appears to us very red. This was strikingly illustrated in August, 1883, when the great volcano, Krakatoa, filled all the air of the earth with dust, and when for many weeks the sun at rising and setting was VOL. XXXIX. - 131. |