parts of the sliders, except the figures, are painted black and opake.

The camera obscura, for drawing landscapes, consists merely of one lens, with a mirror to reflect the images on the rough glass placed to draw upon with a pencil.

Obs. 1.-Before I leave this subject, I must recommend the tutor or student, to dissect a bullock's or sheep's eye. By taking off the back delicately, he will see the landscape before the eye beautifully painted on the optic nerve; he will find in front, the cornea; beneath it, the aqueous humour; then the iris; so called from its various colours; the pupil or hole; which opens and shuts to the light; and the chrystalline humour, or lens. Then, the vitreous humour; and then, the optic nerve or net-work ;—a curious and wonderful arrangement!

2. The instantaneous motion of light has given rise to the TELEGRAPH, a modern invention of the greatest social importance, at present limited to purposes of governments; but capable of the highest uses to the community at large.

The telegraph consists of a large frame, in which are placed and worked six shutters, marked a, b, c, d, e, f, by means of ropes pulled in the manner of bellropes. By the various combinations of these shutters, 63 distinct signals may be produced, sufficient to represent the 24 letters of the alphabet, the 10 digits, and various leading words. Such telegraphs are then set up on eminences, at the distance of 8, 10, or 12 miles; and a line of them, by repeating each other's signals, conveys a message from the first station to the last, at the rate of a hundred miles in about five mi

nutes!

20

The hole and telescope at T, is for the observer; who, in clear weather, is constantly on the look-out.

599. Every 100 parts of the atmosphere is cont posed of 28 parts oxygen, and 72 of azote or nitrogen, kept in a gaseous state by caloric or heat.

Atmospheric air is found to weigh in proportion to rain water, as 12 to 10,000, and to oxygen gas, as 5 to 6; and to hydrogen, as 15 to 1; a cubit foot of it weighing 525 grains; or one ounce and a quarter, nearly.

600. The atmosphere is found to be very elastie; and, in consequence, to press on every side, equal to a weight of 33 feet of water, or 29 inches of mercury; and this elasticity is found to decrease, as we ascend

higher and higher, so as to render the barometer a means of ascertaining heights.

Obs. 1.-This elasticity is equally powerful in a cubic inch of the atmosphere, as in the whole mass; and an inch will raise the mercury in the barometer, as much as the whole atmosphere. One cannot, therefore, but wonder at the quackery, or inconsiderateness of authors, who copy, one after another, the idle nonsense about the atmosphere pressing a man with a weight of 30,000 lbs. ; when, in fact, he is not pressed to the amount of an ounce; all the vesicles of his body being filled with air, which presses outward, at least as much as the atmosphere presses inward, and also upward as well as downward. In fact, in regard to animal and vegetable bodies, the slight gravity of the air is destroyed by its elasticity.

601. Comparing the atmosphere to fleeces of wool laid upon one another, it will be lighter or rarer as we ascend in it; or, in other words, its elasticity will be diminished.

Were it all of uniform density, like water, it would be about five miles high; but the reflection of the sun's rays appears to be affected by it at the height of 44 miles; where it is calculated to be 4,000 times less elastic than on the surface of the earth.

The blue colour of the atmosphere is its natural colour. Its power of reflection produces the universal diffusion of light.

Obs. On the elasticity of air, is founded the invention of the DIVING BELL; by means of which, an operator descends to any depth in water, and remains there for hours together. Weights are placed at bottom to prevent it from turning; and a forcing pipe sends in fresh air, to supply the waste of air from the respiration of the operator.

602. By means of the air-pump, all the air may be drawn out of a large glass-vessel, and a vacuity or vacuum produced; in which a great number of curious experiments may be performed, shewing at once the properties and usefulness of air.

Che figure represents an air-pump on the best modern construction. Che glass-receiver, as it is called, is placed at top; where there is a hole, to let out the air by the action of the pistons seen below.

603. In the aerial vacuum, a feather and a guinea will fall with equal velocity, owing to there being no resistance of the air.

If a bladder, apparently empty, be tied at the neck and left in it, the small quantity of air in the bladder will swell it, and presently burst it.

A bell will cease to sound in vacuo.

The smoke of a candle, having no air to float in, will fall to the botttom by its own weight.

No animal will live, or any light burn, in vacuo.

Obs. A bladder, tied in the same manner, will swell and burst, if laid before a fire; thereby proving the power of heat to rarefy air.

604. Common air may also be compressed, by cold or by mechanical means, into forty thousand times its ordinary space, and still maintain its elasticity; and on this principle is founded the invention of the airgun. Air has a constant disposition to maintain its equilibrium, level, or equal diffusion, like water.

Hence, if a bladder, filled with rarefied air, burst, an explosion takes place, from the rushing of the surrounding air to fill the space.

The same principle is the cause of all wind, which may be traced to some local expansion or compression of air by heat and cold; thus, also, smoke is carried up chimneys.

Obs. It is evident, that the density of bodies must be diminished by expansion; and in the case of fluids and gases, the parts of which are mobile, many important phenomena depend upon this circumstance. If heat be applied to fluids or to gases, the heated parts change their places and rise; and the colder parts descend and occupy their places. Currents are constantly produced in the ocean and in great bodies of water, in consequence of this effect. The heated water rises to the surface in the tropical climates, and flows towards colder ones; thus the warmth of the Gulf-stream is felt a thousand miles from its source, and deep currents pass from the colder to the warmer parts of the sea: the general tendency of these changes, is to equalize the temperature of the globe.

605. One of the principal foreign bodies mixed with, or dissolved in the atmosphere, is the vapour of water which is constantly rising at every degree of heat, provided the force of the vapour already in the atmosphere is not greater than that of vapour at the existing temperature.

By this perspiration of the globe, 36 inches of wa