LEROY C. COOLEY. 117 



rately graduated with the zero at the bottom of the scale. 

 The other tube, destined to hold the pressure- column of 

 mercury, was about seventy feet high. It was built up 

 of successive lengths, joined with the greatest care to 

 avoid leakage, and supported alongside a mast by 

 means of staples, each length being at the same time 

 counterpoised by weights attached and suspended over 

 pulleys. The two tubes were joined together by a cast- 

 iron pipe, which was also in communication with a 

 forcing pump. 



Mercury was driven out of this forcing pump in both 

 directions equally, crowding the air in the air-tube and 

 rising freely in the other. To keep the air from being 

 heated by compression, the air tube w T as enclosed in a 

 jacket through which a stream of water slowly circu- 

 lated. The mercury rose to greater and greater heights, 

 the air was crowded into smaller and smaller space, 

 while the readings of the scales from time to time gave 

 the numerical values of the volumes, and the corre- 

 sponding pressures up to twenty-seven atmospheres. 



At this pressure the air should by Boyle's law occupy 

 just one twenty-seventh of its original volume. The 

 actual reading of the scale showed it to be somewhat 

 less than this, and air did accordingly seem to be more 

 compressible than the law required that it should be. 



Nevertheless, the departures from the law were very 

 small, and, remembering the difficulties they had expe- 

 rienced in their attempts to avoid error, and the slight 

 uncertainty which they could not banish from their 

 judgment in reading the minute changes on the meas- 

 uring scale, Dulong and Arago decided to regard them 

 as falling within the limits of experimental error. It 

 was, therefore, generally conceded that in the case of air 

 the rigorous truth of Boyle's law had been established 

 by their experiments. 



