above the level of the pool M, does not exceed thirty-two feet, for to that height only will the water rise in the syphon by the pressure of the atmosphere; and provided that the end of the syphon at L descends a little way beyond the level of the pool at M,-in which case, when the syphon is filled, the water will rush out at L, so long as any remains in the pond. In the same way may be shown how a cask of liquor may be decanted by a syphon placed in a hole made in its upper side. The use of the syphon might likewise be shown when placed in a reverse position, as in Fig. 5, when it may be applied to the purpose of conveying water Fig. 5.

R

from a fountain at R, along a hollow or valley to a house, S, at the same height on the other side of the valley; and however deep or broad the valley may be, the water may in this manner be conveyed, provided the syphon is sufficiently strong near its lower parts to sustain the perpendicular pressure of the water.

The following simple and interesting experiment might be exhibited to show the effects of the expansion of air. Procure a common Florence flask, F G, Fig. 6, and pour into it a large wine-glassfull of water; then take a tube, I H, bent at the top, H, like a small syphon, and fasten it air-tight into the mouth of the flask, I, so that its bottom may be immersed in the water at K, but not touching the bottom of the flask. Then immerse the flask into a vessel of very hot water, when, in consequence of the expansion of the air in the flask, the water at K will be forced up into the tube I H, where it is received into a wine-glass at H. Holding the wine-glass, into which the water is now received, at the end of the tube, as represented in the figure, take the flask out of the hot water, and plunge it into another vessel full of cold

water, and the water in the wine-glass will be thrown back into the bottom of the flask, by the pressure of the atmosphere on its

surface at H. The flask may then be again immersed in the hot water, when the water at its bottom will be thrown up into the wine-glass as before, and the operations may be repeated as often as judged expedient. This experiment, when dexterously performed, seldom fails to produce a pleasing effect upon the spectators, especially when the water is tinged with a red colour, by means of the sulphuric or any other acid dropped into an infusion of red cabbage.*

In arranging and performing such simple experiments as those above stated, it is expedient that the teacher or operator should know how to cut phials and glass tubes, and to form syphons. The neck of a common phial may be cut off, so as to form a tube, by slightly indenting a portion of the circumference with the sharp edge of a common file, and then, with the point of a hot iron, beginning at the indention, go round the circumference of the phial, and the head will at once be separated from the body. Otherwise, tie a thread which has been steeped in turpentine or spirits of wine, firmly round the mouth of the phial, then set fire to it, and the operation is performed. In the same manner, long glass tubes may be cut into any lengths. If the tubes be of a small diameter, it is only requisite to indent them with a file at the point where they are intended to be cut, and then, holding one end of the tube in the left hand, give a blow with the right on the other end, and the tube will snap asunder.-To bend a glass tube into the form of a syphon: Put the tube through the bars of a common grate, when the fire is burning clear; let the part of the tube which is to be bent be in the centre or hottest part of the fire; take hold of the tube at both

The science of Optics affords scope for many delightful and interesting experiments; but some of its instruments are very expensive. I shall therefore state only a few simple exhibitions and experiments which can be made at a trifling expense. Before the teacher can illustrate any of the principles of this science by experiment, it will be requisite that he provide himself with a few convex lenses, some of short and others of pretty long focal distances. For example, double or plano-convex glasses, inch, 1 inch, 3 and 4 inches, focal distance, which may be made to illustrate the construction of a compound microscope, as I have elsewhere shown in my work, "On the Improvement of Society." Also lenses, from 3 to 6 or 8 feet focus, to illustrate the construction of a telescope, and the nature of a camera obscura; and two or three concave mirrors for illustrating some of the phenomena of reflection. The principle on which a compound microscope, a solar microscope, and a magic lantern or phantasmagoria, are constructed, may be shown by one easy experiment. Let A, Fig. 1, represent a convex glass, suppose six inches focal distance, and B the flame of a candle. Hold the glass, A, at a little more than six inches from the candle, and on an opposite wall will

с

Fig. 1.

E

B

D

be formed a large magnified image of the candle, CED. This image will be inverted, and larger than the flame of the candle in proportion as the distance A E, from the glass to the wall, ex

ends, and when it begins to melt near the middle, gently bend it with both hands, in the form which is wanted, and then remove it from the fire. A little experience will render such operations quite easy and efficient for the purpose intended. If a small bend only at one end of the tube is required, that end may be put into the fire till it begin to melt, then take hold of it gently with a pair of tongs, and bend it in the form required with the rich; hand.

ceeds the distance A B, from the glass to the candle. Suppose the distance A B to be exactly 6 inches, and the distance A E to be 7 feet or 84 inches, then the image of the candle will be magnified in the proportion of 6 to 84, or 14 times. In this experi ment the candle represents the object to be magnified in a compound microscope, A the object-glass, and C D the image formed by the lens, which is magnified a second time by the eye-glass of the microscope. In reference to the solar microscope, the candle represents the small object to be magnified, and C D its magnified image on a white wall or screen; and in reference to the magic lantern, or phantasmagoria, the candle represents the figures painted on the sliders, A the convex lens which throws the image of the figures on a screen, and C D the magnified image of the painted figures. In all these instruments, the principle on which the objects are magnified is precisely the same; the size of the image is always in proportion to its distance from the lens by which it is formed; but as the image is enlarged it becomes less brilliant and distinct, and therefore there is a proper medium which must be fixed upon as to the distance between the lens and the screen on which the image is thrown; but a skilful teacher will always know how to modify such circumstances.

The nature of a telescope and of the camera obscura may be illustrated as follows:-Fix a lens of 4, 5, or 6 feet focus, in a hole made in a window-shutter; darken the room, so that no light can enter but through the lens.* If its focal distance be 5 feet, or 60 inches, a white screen placed at that distance will receive the image of the objects without, opposite the glass, where they will be beautifully depicted in all their forms, colours, and motions, in an inverted position, forming a kind of living picture. This exhibition never fails to excite the admiration of the young. If now, a lens, about 2 inches focus be placed 2 inches beyond the image thus formed, and the screen removed-in looking through this lens, the objects will appear magnified in the proportion of 2 inches to 60, that is, 30 times; and as the image was inverted, so the object, as seen through the glass, will appear as if turned upside down. This is perhaps one of the best modes of explaining the principle of a refracting telescope, and the reason why the object appears inverted, when viewed with a single eye-glass. The same thing may be partly shown by a common telescope.

A lens is a round piece of glass, ground either concave or convex. All lenses that magnify objects, are convex, or thicker in the middle than at the edge, such as common magnifiers, reading-glasses, and the glasses used in microscopes and telescopes, except the Galilean perspective, in which the eye-glass is oncave.

Having taken out all the eye-glasses, except the one next the eye, adjust the telescope to distinct vision, and all the objects seen through it will appear as if turned upside down. The manner in which the image is reversed by the other eye-glasses, and the object made to appear upright, might then be explained. Objects might likewise be exhibited through a telescope, as appearing in different positions and directions. This is effected by means of a diagonal eye-piece, which is constructed in the following manner: Let A B, Fig. 2, represent a convex glass about 2 inches focal distance; CD a plain metallic speculum, of Fig. 2.

E

an oval form, well polished, and placed at half a right angle to the axis of the tube; and E F, another convex lens, 2 inches focus. The centre of the speculum may be about 14 inch from A B, and about inch from E F. The rays proceeding from the lens A B, and falling upon the speculum, are reflected in a perpendicular direction to the lens EF, where they enter the eye, which looks down upon the object through the side of the tube. When this eye-piece is applied to a telescope, with the lens E F on the upper part of it, we look down upon the object as if it were under our feet. If we turn the eye-piece round in its socket a quarter of a circle towards the left, an object directly before us in the south will appear as if it were in the west, and turned upside down. If from this position, it is turned round a semicircle towards the right, and the eye applied, the same object will appear as if it were situated in the east; and if it be turned round another quadrant, till it be directly opposite to its first position, and the eye applied from below, the object or landscape will appear as if suspended in the atmosphere above us. Such experiments, when accompanied with proper diagrams, and an explanation of optical principles, may easily be rendered both entertaining and instructive.

A camera obscura, on a larger scale, and on a different plan from that alluded to above, might be erected on the top of every school-house, which is constructed with a flat roof, as formerly