EXPERIMENTS ON ANEROID BAROMETERS 



351 



3. Sxiddcn changes of jircsstirc. — For the purpope of investigatiiif;^ the influence 

 of sudden changes of j)ressure upon the indications of aneroids, 1 ai)])lieil to some 

 of the best-known makers of these instruments for the loan of several, and through 

 their courtesy in lending me a suflicient numlier, and for a sutHciently long time, 

 1 have been enabled to investigate this influence at some length. In the fol- 

 lowing experiments the instruments were, to begin with, susjiended vertically, at 

 the usual atmospheric pressure. They were tap})ed before being read. The pres- 

 sure was then lowered an inch, and the instrument allowed to remain 10 minutes 

 at this pressure before being read, after having been again well tapped. The 

 l)ressure was thus reduced an inch every time, being allowed to remain 10 minutes 

 at each stage ; the instrument was always well tapped before being rc^ad, by 

 means of an arrangement contrived for this purpose by Mr. II. Beckley. The 

 exhaustion was carried downwards to 19 inches, and the iiistrument was allowed 

 to remain an hour and a half at its lowest pressure; the air was then admitted aii 

 inch at a time, the previous arrangement as to time and tapjiing being followed. 



Separating the. results of the experiments into tw^o sets, one comprising largo 

 (four inch to four and a half inch) aneroids, and the other small instruments, wo 

 find the "tnean down coiTCCtion to be as follows, each aneroid being supposed 

 riffht at 29 inches: 



We see from these results, says Dr. Stewart, that if aneroids, right to begin 

 with, be subjected to a decrease of pressure similar to that to v»hich they were 

 subjected in these experiments — 



1. That a well-constructed large aneroid will not go far wrong down to 24 

 inches, but after that pressure its reading will be considerably lower than that 

 of a standard biu-ometer, so that a large positive correction will have to be applied. 



2. That small aneroids are less trustworthy than large ones, and probably 

 cannot be trusted below 26 inches. 



3. That if previous experiments are made upon an aneroid, we are enabled 

 by this means to obtain a table of corrections which, when applied to future 

 oi»servati(jns with the same instrument, will most probably present us with a much 

 better result than had we not verified our instrument at all, and that by this 

 means we may use our instrument down to 19 inches with very good results. 



Keadings of these instruments under increasing pressure, after remaining an 

 hour and a half at the lowest reading, w'ere recorded. 



The mean coiTections for up readings are exhibited in the following table, 

 each aneroid being supposed right at 19 inches: 



For two largo aneroidg 



For foui Binall aneroids 



19 in. 



.00 

 .00 



20 in. 



+.03 

 .00 



21 in. 22 in. 



+ .03 

 +.01 



+ .03 



+.02 



23 in. 



+.03 

 —.01 



24 in. 25 in 



+.02 

 —.01 



+ .01 

 —.02 



26 in. 



.00 

 -.04 



27 in. 



—.03 

 —.07 



28 in. 



29 in. 



—.06 

 —.10 



-.08 

 —.15 



30 in. 



—.11 

 —.16 



We may learn from these results, says Dr. Stewart, that if aneroids which have 

 been sulij<'cted for at least one hour and a half to the lowest pressures which they 

 register, liave the pressure increased by means of the gradual introduction of air 

 into the receiver, after the manner already described — 



1 . That a well-constructed large aneroid will not go far wrong for about 8 inches 

 above the lowest preesure. 



