Ixvi 



INTRODUCTION TO PNEUMATICS. 



A shrivelled apple placed within a receiver becomes plump from the 

 expansion of the air within it, as soon as the pressure of the external air is 

 taken away, and shrinks to its former dimensions when the air is again 

 let into the receiver. If two bodies be placed so close together that there 

 is absolutely no air between them, to counterbalance by its elasticity the 

 pressure of the air on their outer surfaces, that whole pressure resists 

 their separation. It is thus that a stone may be raised by a string fixed to 

 a piece of moistened leather pressed close upon it ; and thus also that in 

 the experiment detailed in page 6, the pressure of the atmosphere, as 

 well as the cohesion of the metallic hemispheres, prevents their separa- 

 tion. 



A column of air reaching to the top of the atmosphere, and whose base 

 is a square inch, weighs 15 Ibs. when x the air is heaviest. The rule that 

 fluids press equally in all directions applies to elastic fluids as well as to 

 liquids : therefore every square inch of our bodies sustains a pressure of 

 15Jbs., and the weight of the whole atmosphere may be computed by 

 calculating the number of the square inches on the surface of the earth, 

 and multiplying them by 15. 



The weight of a small quantity of air may be ascertained by exhausting 

 the air from a bottle, and weighing the bottle thus emptied. Suppose 

 that a bottle, six cubic inches in dimension, weighs two ounces; if the air 

 be then introduced, and the bottle re-weighed, it will be found heavier by 

 two grains, shewing that six cubic inches of air (at a moderate tempera- 

 ture) weighs about two grains. In estimating the weight of air, the tem- 

 perature must always be considered, because heat, by rarefying air, ren- 

 ders it lighter. The same principle indeed applies, almost without 

 exception, to all bodies. In order to ascertain the specific gravity of air, 

 the same bottle may be filled with water, and the weight of six cubic 

 inches of water will be 1515 grains : so that the weight of water to that of 

 air is about 800 to 1. 



A barometer is an instrument which indicates the state of the weather, 

 by shewing the weight of the atmosphere. It is extremely 

 simple in its construction, and consists of a glass tube, 

 A B (fig- 1), about three feet in length, and open only at 

 one end. This tube must first be filled with mercury, then 

 stopping the open end with the finger, it is immersed in a 

 cup, C, which contains a little mercury. Part of the mer- 

 cury which was in the tube now falls down into the cup, 

 leaving a vacant space in the upper part of the tube, to 

 which the air cannot gain access. This space is therefore 

 a perfect vacuum ; and consequently the mercury in the 

 tube is relieved from the pressure of the atmosphere, 

 whilst that in the cup remains exposed to it: therefore the 

 pressure of the air on the mercury in the cup supports that 

 in the tube, and prevents it from falling: thus the equili- 

 brium of the mercury is destroyed only to preserve the 

 general equilibrium of fluids. This simple apparatus is 

 all that is essential to a barometer. The tube and the cup 

 or vase are fixed on a board, for the convenience of sus- 

 pending it; the board is graduated for the purpose of 

 ascertaining the height at which the mercury stands in ihe 

 tube ; and the small moveable metal plate serves to show 

 that height with greater accuracy. The weight of the 

 atmosphere sustains the mercury at the height of about 



Fig. 1. 



