464 



DR. C. CHREB, EXPERIMENTS ON ANEROID BAROMETERS 



others, Nos. 52 and 55 were made at a temperature of 50 F., Nos. 53 and 54 at a 

 temperature of 81 F. Use was made of the room employed for testing chrono- 

 meters, the temperature of which is readily controlled. Information as to tempe- 

 rature effects is also obtainable from the other special experiments, because, being 

 taken at all seasons of the year in an ordinary room, they answered to considerably 

 varied temperatures. 



The investigation of the influence of temperature on the descending readings is 

 complicated by the fact that the index error of an aneroid is a somewhat variable 

 quantity. To get rid, so far as possible, of this uncertainty, I subtracted from all the 

 readings a constant equal to the error observed at 30 inches. This is eqxiivalent to 

 making the instrument correct at the start of each experiment. The excesses of the 

 readings so modified over the true pressures at lower points of the range I shall call 

 the corrected errors. 



Table XVIII. shows the algebraic excess of the corrected errors from the mean of 

 experiments 52 and 55 over the corrected errors from the mean of experiments 

 53 and 54. 



TABLE XVIII. Corrected Errors temperature 50 F., less Corrected Errors 



temperature 81 F. 



There is here a very decided fall of reading accompanying rise of temperature, and 

 the fall appears to increase directly as the pressure interval measured from 30 inches. 



23. Similar conclusions follow from the experiments as a whole. For the two 

 aneroids Nos. 2 and 3 there were 22 experiments over the range 30-21 inches at the 

 normal rate. These I divided into two groups, the 11 colder in the one, the 11 

 hotter in the other, and found the mean corrected errors for the two groups separately. 

 Aneroid No. 4, for some reason, changed its behaviour during a long rest between 

 experiments Nos. 46 and 47. Prior to the change, however, there were 25 experi- 

 ments in which the pressure was lowered to or below 21 inches. Twelve of these 

 with temperatures from 77 to 71 formed the hotter group. The colder group 



