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 12 1/2 inches, and multiplying this by 15 lbs. we obtain 187 1/2 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 2160 lbs. the pressure 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 Magdeburg, 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 immediately manifest. The law of compression within certain limits is exceedingly simple, and may be easily verified. Let a long glass tube be closed at one end, as in the accompanying representation. 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 b i will be enclosed and prevented from escaping. If now more mercury be poured into the longer leg till it rise to d in the shorter, the height of mercury in the longer leg above the level c d will be found to be about 10 inches, which is 1/3 of the usual atmospheric pressure. The whole pressure upon the column di being made up of the pressure of the external air, together with that of the 10 inches of mercury, will be 4/3 of the atmospheric pressure, and the space now occupied by the air is 3/4 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 pressure, 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: -

 Compressing force ........................... 1 3/2 4/2 5/2 6/2 Spaces occupied........................... 1 2/3 2/4 2/5 2/6

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 proportional 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, etc.

The elasticity of air by the removal of the pressure gives rise to a variety of entertaining experiments. If a bladder containing a small portion of air be placed under the receiver of an air pump, while the air is exhausting the bladder will be observed to expand till it appears fully blown; on the reentry of the air the pressure will immediately reduce the included air to its primitive dimensions, and the sides of the bladder will collapse. At the larger end of an egg there is a bubble of air between the shell and the inner skin: if a hole be made at the smaller end, and the egg be placed with the hole downwards in a wine-glass, under the receiver of an air pump, as soon as the air is begun to be withdrawn, the air within the egg will expand and force out the contents into the glass. When the air re-enters, by careful management, the whole may be forced into the shell, so as to have its original appearance. Upon this principle fountains may be contrived. If a glass or other vessel similar to the one here represented, having a tube reaching nearly to the bottom, be half filled with water, and then placed under a tall receiver on the pump plate, the action of the pump commences, the air in the part a not being able to escape, expands itself as the external pressure is removed, and forces the water before it up the pipe, so as to form a continuous stream till the level of the water reaches the lower end of the tube.

If the air in the part a could be compressed so that its elasticity might exceed that of ordinary atmospheric air, the fountain would act without being placed under a receiver. For this purpose a condensing or compressing syringe would be necessary to force air into the upper part of the vessel. The compressing syringe differs but little from one of the barrels of the air pump. It has, however, no valve in the piston, but one at the end b, opening outwards, and which may be easily formed by tying over the hole a small piece of oiled silk. When this apparatus is to be used, the end b is screwed into the mouth of the vessel into which air is to be forced; and the piston being then raised above the hole a in the side, the syringe becomes filled with air: the piston is then depressed, and the air is, by its descent, forced into the vessel, and from which it cannot return on account of the valve at b opening only downwards. The piston is again raised till above the hole, and another barrel full of air is injected into the receiver. This process may be continued till the air is considered of sufficient density, which may be easily ascertained by knowing the proportionate capacity of the syringe and the receiver.

If the receiver contain twelve times as much as the syringe, twelve changes of the syringe will be necessary to double the density of the air. It is, perhaps, scarcely necessary to remark that the receiver must be strong and furnished with a stop-cock or valve, so that when the syringe is separated from it the air may not escape. For further information on this science, see Atmosphere, Air, Barometer, etc.