INTRODUCTION TO PNEUMATICS. Ixvii 



29J inches ; but the exact height depends upon the weight of the atmos- 

 phere, which varies much according to the state of the weather. The 

 greater the pressure of the air on the mercury in the cup, the higher it 

 will ascend in the tube. The air therefore generally is heaviest in dry 

 weather, for then the mercury rises in the tube, and consequently that 

 in the cup sustains the greatest pressure ; and thus we estimate the 

 dryness and fairness of the weather by the height of the mercury. We 

 are apt to think the air feels heavy in bad weather, because it is less 

 salubrious when impregnated with damp. The lungs, under these cir- 

 cumstances, do not play so freely, nor does the blood circulate so well : 

 thus obstructions are frequently occasioned in the smaller vessels, from 

 which arise colds, asthmas, agues, fevers, &c. 



As the atmosphere diminishes in density in the upper regions, the air 

 must be more rare upon a hill than in a plain ; and this difference may 

 be ascertained by the barometer. This instrument is so exact in its 

 indications, that it is used for the purpose of measuring the height of 

 mountains, and of estimating the elevation of balloons. Considerable 

 inconvenience is often experienced from the thinness of the air in such 

 elevated situations. It is sometimes oppressive, from being insufficient 

 for respiration ; and the expansion which takes place in the more dense 

 air contained within the body is often painful : it occasions distension, 

 and sometimes causes the bursting of the smaller blood-vessels in the 

 nose and ears. Besides, in such situations, you are more exposed 

 both to heat and cold ; for though the atmosphere is itself transparent, 

 its lower regions abound with vapours and exhalations from the earth, 

 which float in it, and act in some degree as a covering, which preserves 

 us equally from the intensity of the sun's rays, and from the severity of 

 the cold. 



Now since the weight of the atmosphere supports mercury in the tube 

 of a barometer, it will support a column of any other fluid in the same 

 manner ; but as mercury is the heaviest of all fluids, it will support a 

 higher column of any other fluid ; for two fluids are in equilibrium, when 

 their heights vary inversely as their densities : as, for instance, if a cubic 

 foot of one fluid weighs twice as much as a cubic foot of the other, a 

 column of the first ten feet in height will weigh as much as a column of 

 the other twenty feet in height. Thus the pressure of the atmosphere, 

 which will sustain a column of mercury of twenty-nine inches, is equal to 

 sustaininn- a column of water of no less than thirty-four feet above its level. 

 The weight of the atmosphere is therefore as great as that of a body of 

 water surrounding the globe of the depth of thirty-four feet ; for a column 

 of air of the height of the atmosphere is equal to a column of water of 

 thirty-four feet, or one of mercury of twenty-nine inches, each having the 

 same base. 



The common pump is constructed on this principle. By the act of 

 pumping, the pressure of the atmosphere is taken off one part of the sur- 

 face of the water : this part therefore rises, being forced up by the pressure 

 communicated to it by that part of the water on the surface of which the 

 weight of the atmosphere continues to act. The body of a pump consists 

 of a large tube or pipe, whose lower end is immersed in the water which 

 it is designed to raise. A kind of stopper, called a piston, is fitted to 

 this tube, and is made to slide up and down it, by means of a metallic 

 rod fastened to the centre of the piston. 



The various parts of a pump are delineated in jig. 2 (next page). A B is 

 the pipe or body of the pump ; P the piston ; V a valve,, or little door iu 



F2 



