Another experiment illustrative of the same fact is termed the guinea and feather experiment. A long receiver is placed upon the pump plate, and a guinea and a feather are attached at the top to a little piece of apparatus by which they may be disengaged at the same instant. While the receiver is full of air, the guinea reaches the pump plate before the feather, but when the air is taken from the receiver, the guinea and feather fall with exactly the same velocity.
Since air is fluid, it will manifest the common properties of fluids; as, for example, pressure. If a small vessel, similar to the one here represented, be placed over the hole in the pump plate, and the hand placed closely over the top, when the pump is worked, the hand will be held-firmly on the glass by means of the downward pressure. In the same way, the glass receivers are held firmly on the pump plate. If a bladder be made wet, and tightly stretched over the top of the glass, then dried and placed over the hole of the air pump, as soon as the pump is worked, the bladder will appear concave at the top, and will eventually be burst by the great pressure of the superincumbent air. Another apparatus, admirably adapted to evince the great pressure of the air in all direc tions, is what are termed the Magdeburg hemispheres. It consists of two hemispheres of brass, having their edges accurately ground, so that they may fit together, as in the annexed representation. The part a of the lower hemisphere is screwed into the hole of the pump plate, and the air may then be exhausted. If then the handle b be screwed on, two persons may endeavour to separate them by pulling in oppo site directions, or they may be suspended, and a weight attached to the lower one. It has been ascertained that the actual amount of the air's pressure is about 15 pounds on every square inch of surface ; hence may be calculated the force with which the hemispheres are held together, or the absolute pressure upon any surface whatever. Let us suppose that the diameter of the hemispheres is 4 inches, then the area of each of the circles in contact with each other will be 12e inches, and multiplying this by 15 lbs. we obtain 187i lbs. as the pressure by which the hemispheres are held together. In the same way we may ascertain the amount of pressure upon the human body. Suppose the outer surface of a middle-sized man to be about 14 Square feet, then multiplying this by 21601bs. the pres sure on a square foot, we obtain 30,240 lbs. as the pressure upon the body of an individual of moderate size. If the barometer should fall an inch, which
it frequently does before rain, we are released from a pressure of upwards of 1000 lbs. : this, by diminishing the tension of the different parts of the body, is sufficient to account for that languor which is commonly complained of in bad weather. This apparatus was originally designed by Otto Guericke, of Magde burg, and was constructed on so large a scale that several horses were required to separate the hemispheres.
The ordinary or natural state of the air (as we are in the habit of calling it,) is a compressed state ; if we attempt to alter it either by further compression, or by taking off the pressure, the elasticity or repulsion of the parts is imme diately manifest. The law of compression within certain limits is exceed ingly simple, and may be easily verified. Let a long glass tube be closed at one end, as in the accompanying represen tation. The longer leg may be 30 or 40 inches in length, the shorter 4. Suppose the tube placed in an upright position, and a little mercury poured into.it up to the level a b, then a cylinder of air bi will be enclosed and prevented from escaping. If now more mercury be poured into the longer leg tall it rise to d in the the height of mercury in the longer leg above the level cd will be found to be about 10 inches, which ie I. of the usual atmospheric pressure. The whole pressure upon the column d i being made up of the pressure of the external air, together with that of the 10 inches of mercury, will be I. of the atmospheric pressure, and the space new occupied by the air ie of the original space. If more mercury be added, so that the column may be 30 inches high, the whole pressure will be double the atmospheric pres sure, and the space into which the air will be compressed is one-half. If we examine the result of a number of trials made in this way, we shall find them as follows:— If we examine these two rows of fractions, we shall find that the lower are the reciprocals of the upper ; whence we see that the spaces occupied by the compressed air are inversely as the compressing forces. But as the density is inversely as the spaces occupied, it is evident that the compressing force is pro portional to the density ; and further, since the elasticity of the included air is proportional to the compressing force, it is also manifest that the elasticity is as the density, that is, if the density be doubled or tripled, the elasticity will be doubled or tripled, &c.