its opening is completely closed by heating that end of the glass tube with charcoal or a blowpipe, so as that it may be twisted round; thus entirely excluding the external air. This operation is called hermetically sealing the tube, The glass tube being prepared in this manner, is attached to a graduated scale, which is made by first plunging the thermometer into a mixture of pounded ice or snow and sal-ammoniac; the point at which the quicksilver settles down, is marked O; which is the beginning of the scale. It is afterwards immersed in boiling water, and the point to which the quicksilver rises is marked 212°. The intermediate part of the thermometer is divided into an equal number of degrees. The quicksilver in the tube will rise or fall in proportion to the presence or absence of caloric in the surrounding medium.

Another machine, by which a great number of interesting experiments are performed, is the air-pump. It was invented in 1654, by Otto Gueric, a German, and by means of it a glass or any other vessel may be deprived of the atmospheric air, by which it had been filled. It pumps out the air by means of pistons working in brass trunks, or large tubes. A metal plate is connected with the barrels, by an orifice in its centre, over which is put a piece of wetted leather having a hole in it to correspond with the orifice in the plate. Upon this is inverted a glass bell, and under the bell is placed the substance or vessel from which the air is to be pumped out through a tube. When the air is exhausted from the inverted glass bell, it is fastened down so

tight to the plate by the external air, that it cannot be removed without great force, until the air is let into the bell again, when it is easily taken up.

The air-gun is an instrument which shows the elasticity and compressibility of air, in a very striking manner. In this instrument a quantity of air is condensed or pressed very close, by various contrivances, in such a manner, that the condensing power being suddenly removed, a bullet will be sent to a considerable distance, with little or no noise, but with great force, by the spring or elasticity of the air returning to its natural bulk. In this manner the elasticity of the air will drive a ball with such violence as to pierce an oaken board, half an inch thick, at the distance of 26 yards.

Balloons are machines filled with a gas considerably lighter than the atmospheric air, in consequence of which they rise in that air. In 1766, Cavendish discovered that inflammable air, or hydrogen gas, is seven times lighter than common air; and soon after this, Black made soap bubbles ascend by filling them with inflammable air. In 1783, two brothers, named Montgolfier, paper manufacturers, at Annonay, in France, caused a large bag to ascend to a very great height, by filling it with rarefied air, produced by burning straw and chopped wool. This gave

the idea to the philosophers at Paris, of inflating a globe of silk with hydrogen gas. The globe, when filled, rose to the height of 3123 feet, in two minutes.

The first person who ventured to ascend into the atmosphere, in a car or boat attached to one of these machines, was Pilatre de Rozier, who afterwards lost his life by the balloon's catching fire. Since that period many persons have ascended to great elevations with balloons. The inflammable air balloon is now preferred to the Montgolfier, or rarefied air-balloon; because the latter is always in danger from the fire which must be attached to it, in order to rarefy the air within the globe. The balloon is generally made of silk, varnished with a solution of elastic gum. If the person carried up with it wish to mount higher, he throws out some ballast. If he choose to descend, by opening a valve he lets some of the gas escape gradually. The car, or boat in which the aeronaut sits, is generally suspended by a netting, which is thrown over the whole of the silken globe.

An instrument has been invented for measuring the quantity of rain that falls. This is called a rain-gauge. It consists of a funnel, whose diameter is twelve inches, attached to a tube of four inches diameter. In the tube is a floating graduated index, which rises as the rain falls into, and rises in, the funnel and the tube. By this instrument, the nine hundredth part of an inch of rain may be estimated.

Another instrument, called a hygrometer, is used for measuring the degrees of moisture. This is constructed of various materials, and in different ways. The most common and simple hygrometer, is a piece of whipcord, or catgut,

fastened at one end to a block of wood or metal, and stretched over several pulleys; while at the other end are a small weight and an index, which move up and down a graduated scale.

QUESTIONS.

What are the objects of the science of pneumatics? What are the properties of air?. How is air proved to be material? How is air proved to have weight or gravity? Why is air said to be a fluid? How does it appear that air has impulsive force? Why is air transparent? Has the air different degrees of weight? What is the proportion of the pressure of a column of air, to that of a column of quicksilver? What is the pressure of the atmosphere upon the human body, and how is it that the great weight does not crush the human body? What are the beneficial effects of this pressure of the air? How far is the atmosphere supposed to extend from the surface of the earth? What is the barometer? What is the thermometer? What is the airpump? What is the air-gun? What is the air-balloon? What is the rain-gauge? What is the hygrometer?

CHAP. XLII.

PNEUMATICAL EXPERIMENTS.

BODIES cannot move in the atmosphere without displacing a portion of it proportionable to their expansion. This requires force, and the resistance of the air always diminishes the velocity of bodies moving in it. This is the reason why light bodies, such as feathers, fall more slowly than bodies which have more bulk or weight, but less extension of surface. Their moving force is very small, and can, therefore,

displace a large quantity of air, only with a very small velocity. But if the same bodies be dropped in a vacuum, where there is no air to be displaced, they fall with the whole velocity competent to their gravity. Thus, a guinea and a feather being let fall in the exhausted receiver of an air-pump, reach the bottom at the same instant.

By means of the air-pump, the air can be completely extracted from a dry vessel, so as that the precise weight of the air which filled it, may be estimated. Dr. Hook found that 114 pints of air, weighed 945 grains, that one pint of water weighed 8 ounces; which gives for the specific gravity of air,, very nearly. If a small beam be suspended within a receiver; to one end of which is attached a thin glass, or copper ball, closed in every part, and balanced by a small piece of lead hung on the other arm, and the receiver be then exhausted of its air; as the exhaustion takes place, the ball will gradually preponderate, that resistance being thus withdrawn, which supported it before; but as the air is readmitted, the ball will regain its equilibrium, the resistance being restored.

When the air is abstracted from the receiver, it is strongly pressed to the pump-plate, by the incumbent atmosphere; and it supports this great pressure in consequence of its circular form.

Place a small receiver, or cupping glass on the pump-plate of the air-pump, without covering the central hole, and cover the small, with a larger receiver. Exhaust the air from it, and then admit it as suddenly as possible. The outer