512 
MR. WELSH ON THE CONSTRUCTION OF A STANDARD 
city for one inch =L— 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 correction for zero-error. From twenty to thirty com- 
parisons are usually made, care being taken that there shall be as nearly as possible 
an equal number of observations 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 correc- 
tion corresponding to any height /^o of the marine barometer is 
H— A+K(^o-^) + (^-T)xO*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. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
At 27-5 
At 28-0 
At 28-5 
At 29-0 
At 29-5 
At 30-0 
At 30-5 
At 31-0 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
-1- 0-001 
0-000 
— 0-001 
— 0-001 
— 0-002 
— 0-003 
— 0-004 
— 0-005 
“ When the sign of the correction is -f, the quantity is to be added to the observed 
reading ; and when — , to he 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. It was originally 
mounted on an independent support, but this was found to be too unsteady for exact 
observation. It was accordingly removed from its support, and mounted between 
brackets attached to the quadrant wall. The scale of this instrument has been com- 
pared with the Kew Standard Scale, both in the horizontal and vertical positions ; in 
the former, by observation of both scales by fixed micrometer microscopes, and in 
