416 COMPARISONS OF TEE ANEROID BAROMETER, appendix. 



prolonged periods. [The balloon test is only a repetition of the air-pump 

 test. In the former case the instruments are exposed to a natural, and in 

 the latter case to an artificial diminution of pressure ; and if the duration of 

 time is equal in each case the results ought to correspond exactly.] 



G. That very material errors may be fallen into by regarding ' a good 

 return ' at the level of the sea as a proof of correct working, at low pressures, 

 of aneroids of the present construction. 



H . That for the detection of such errors as aneroids (of the present con- 

 struction) will exhibit when subjected to low pressures for a length of time, 

 aneroids should be subjected artificially to similar pressures for a long 

 period. 



