354 Hydrostatics. 



is necessary to recur to certain experiments relating to the den- 

 sity of the air under different pressures and at different tempe- 

 ratures. 



Fig.230. 468. Take a recurved glass tube ABC. open at the extremity 

 A and closed at the other extremity C ; pour into it a quantity 

 of mercury just sufficient to fill the bended part up to the hori- 

 zontal line DE, so that the air confined in the shorter branch 

 CE may be neither more nor less pressed than that contained 

 in the longer branch AD, which communicates with the atmo- 

 sphere. The mercury being at the same height, therefore, in 

 each branch, and the communication with the external air being 

 cut off, if we introduce, by means of a fine tunnel, more mercury, 

 we shall observe this liquid to stand higher in the branch BA 

 than in the other, whereby the air in EC will be condensed, 

 the compressing force being equal to the difference of the two 

 columns. If the space EC, supposed, for example, to be 4 inch- 

 es, were reduced one half or to E, by the pressure of a column 

 of mercury extending to H, drawing the horizontal line EG, we 

 should find the difference GH of the two columns exactly equal 

 to the height of the barometer at the time of the observation ; so 

 that the air contained in the space CE would be pressed by the 

 weight of the atmosphere incumbent upon H and by the weight 

 of another atmosphere represented by the column GH. A doub- 

 le pressure, therefore, reduces the bulk one half. If we contin- 

 ue to add to the weight by pouring in more mercury till the 

 confined air is condensed to F' or to one third of the original 

 space, we shall find the additional quantity necessary to this 

 effect the same as before, that is, the column GH' will be 

 equivalent to two atmospheres. Thus a triple pressure reduces 

 the bulk of the confined air to one third the space. We might 

 continue to increase the weight, and we should in every instance 

 obtain results agreeable to the same general law. 



So, on the other hand, by diminishing the natural pressure 

 exerted upon any portion of air, we shall still find the bulk in- 

 versely proportional to the pressure. Let the tube ABC be 

 supposed to have a bore not exceeding one tenth of an inch. 

 A drop of mercury being introduced at the bend A, if the whole 

 apparatus be placed under the receiver of an air pump, and the 



