446 



CONSTRUCTION OF A STANDARD BAROMETER. 



to a state of rest, and the zero of the standard having been adjusted, the 

 height of the mercury in each of the barometers is observed by the 

 cathetometer. Air is then admitted till the mercury stands at about 

 31 inches, when the same operation is repeated. The length of the 

 graduated scale of the barometer under comparison is then measured 

 by the cathetometer. If A a be the cathetometer readings at the 

 higher pressure of the standard and marine barometers, respectively, 

 B b those for the higher pressure, and if L be the measured length 

 of one inch of the scale of the marine barometer, then the correction 



for capacity for one inch =z h — — ^. In order to avoid the error 



which might otherwise arise from the different capillary actions of the 

 standard tube and that of the marine barometer, it is the practice to 

 make these comparisons only in the forenoon, when the temperature 

 of the room, and consequently the pressure of the air within the 

 receiver, is slowly increasing. 



Besides the determination of the capacity correction, a series of 

 simultaneous observations are made of the marine barometer and the 

 standard, "Newman 34," for the purpose of obtaining the zero error. 

 From twenty to thirty comparisons are usually made, care being taken 

 that there shall be, as nearly as possible, an equal number of obser- 

 vations with the barometer rising and falling; this being necessary in 

 order to eliminate the retardation produced in the movements of the 

 mercury by the contraction of the tube combined with the capillary 

 action. The final corrections at different heights of the mercury are 

 thus deduced from the data now obtained. Let H be the height 

 (corrected for zero error) of Newman 34; h the corresponding height 

 of the marine barometer; T the temperature of Newman 34; and t 

 that of the marine barometer; K being the "capacity" correction; 

 the correction corresponding to any height h of the marine barome- 

 ter is — 



H — A + K ( A — %) + {t — T)x 0.0027. 

 Each barometer, when it leaves the observatory, is accompanied by 

 a statement of its corrections, of which the following is a specimen: 



Corrections to the scale readings of marine barometer, B. T., No. 231, by Adie, 



London. 



Inches. 

 At 27.5 

 -f 0.001 



Inche*. 

 At 28.0 

 0.000 



Inches. 

 At 28.5 

 — 0.001 



Inches. 

 At 29.0 

 — 0.001 



Inches. 

 At 31.0 

 — 0.005 



When the sign of correction is -{-, the quantity is to be added to 

 the observed reading; and when — , to be subtracted from it. The 

 corrections given above include those for index error, capacity, and 

 capillarity. 



III. — Cathetometer. 



The cathetometer hitherto employed was made by Mr. Oertling, 

 of London, on the plan of that used in the experiments of M. Regnault. 



