EXPERIMENTS ON ANEROID BAROMETERS, ETC. 353 



From all these cspcrimonts, Dr. Stewart concludes as follows : 



A £;oo(l aneroid of large size may be corrected for temperature by an optician, 

 BO that the residual correction shall l>e very small. 



If an aneroid, correct to conuuence witli, be used for a balloon or mountain 

 ascent, it will be tolerably correct for a decrease of about six inches of pressure. 



A hirge aneroid is more likely to be correct than a small one. 



The range of correctness of an instrument used for mountain ascents may be 

 increased by a previous -verification, a table of corrections being thus obtained. 



If an aneroid has remained some time at the top of a mountain, and be sup- 

 posed correct to start -with, then it will p;ive good results for about eight inches 

 of increase of pressure. 



A large aneroid is more likely to be correct than a small one 



If the aneroid has been previously verilied, it is likely to give a better result. 



After being subjected to sudden changes of pressure, the zero of an aneroid 

 gradually changes, so that under such circumstances it ought only to be used as 

 a dillerential and not as an absolute instrument, that is to sa}', used to determine 

 the distauce ascended, making it correct to begin with, or to ascertain the dis- 

 tance descended, making it correct to begin with, it being understood that the 

 instrument caught to be quiescent for some time before the change of pressure is 

 made. 



[I have subjected many aneroids to the test of a sudden diminution of pressure 

 under the receivers of an air-pump, but have never found one of which the index 

 would retiu-n to the same point when the original pressure was restored. I have 

 not, however, had recourse to tapping, which Mr. Stewart has found essential. 

 The instrument in its present condition cannot be relied on to indicati^; absolute 

 pressure, though it ma}' be used in many cases with good eflcct in determining 

 diflerences of pressure of limited extent. — J. H.J 



23 S 



