1841.] Newma7i's improved Portable Barometer, ^c, 961 



The standard Barometer of the Observatory, with which the compari- 

 sons were made, is by Newman, the cistern measures 6 inches in height 

 and three in diameter over all, and about 2^ inches inside, the tube is 

 5.54 inches in diameter inside^ and the scale and vernier are of platinum, 

 the former being attached to a brass rod terminating below in a fine 

 ivory point, which is adjusted to the surface of the mercury in 

 the cistern — the scale reads by means of the vernier to the 500th, and 

 by estimation to the 1000th part of an inch, to which limit all the 

 observations are taken. 



The following are the values given by Mr. Newman of correction for 

 capacity, &c. of the two portable Barometers attached to this Observa- 

 tory :— 



No. 40 — Correction for capacity, -^^ 



For capillary action, 4-042 



Temperature, 60° Faht. 



No. 44 — Correction for capacity, ^ 



For capillary action, -|-031 



Temperature, 60° Faht. 



The Thermometer of No. 44 is broken. 



The comparisons were made in the following manner : the two port- 

 able instruments were hung up on the same pillar with, and one on 

 each side of, the standard Barometer, and a small excess of mercury 

 having been introduced into the cistern, the escape of a portion was 

 allowed to take place by means of the screw below until the height 

 of the column read approximately the same as the standard, making 

 due allowance for the difference in the capillary action of the two 

 tubes ; a perfect coincidence was found impracticable. 



The annexed table exhibits the mean daily results of the compa- 

 risons which were taken, every two hours at the times appointed for 

 the other regular observations, and embrace therefore the period, 

 nearly, of one complete oscillation ; viz. from lOh. 29m. a. m. to lOh. 

 S9m. p. M. inclusive, Observatory mean solar-time, so that each entry 

 is the mean of seven observations, and the mean of the means for No. 40 

 is deduced from 133, and for No. 44 from 1 12 comparisons ; the differ- 

 ences from the standard are as accordant as could have been expected, 

 considering the imperfect nature of the scale upon which the heights 

 are measured, the difficulty of getting a good contact with the surface 



