510 
MR. WELSH ON THE CONSTRUCTION OF A STANDARD 
its index-correction (which, inclusive of capillary action, =+0‘003inch) being first 
applied to the observed readings. 
Comparisons, by means of two portable barometers by AniE, London, were made 
during last summer between the Kew Standard and that of the Observatory of Paris. 
The result of these comparisons was, that the Kew Standard reads higher than the 
Paris Standard by 0 001 inch, no correction being applied to either instrument on 
account of capillary action. 
II. f^erification of Barometers. 
In the best barometers of the present day, a provision is made for adjusting the 
surface of the mercury in the cistern to the zero of the scale at each observation. 
Supposing the tube to be in good order, which is easily ascertained by mere inspec- 
tion, the only source of error in such instruments is to be looked for in the scale. 
The graduation of the scales of all carefully made barometers is performed by means 
of a dividing engine, and is not likely to be inaccurate to any sensible extent within 
the ordinary range of the mercury. If, however, the barometer is intended to be used 
at considerable elevations, or if it should otherwise be considered desirable to examine 
the graduation, the error of the divisions can be readily obtained by measurement 
with the Cathetometer. It frequently happens, however, that the point to which 
the level of the mercury is adjusted is not the true zero of the scale. The error 
arising from this source is of course constant for all heights of the barometer. As 
the capillary action of the tube is also supposed to be constant for the same baro- 
meter, and as it is seldom possible to determine its true amount, it is better to con- 
sider it in connexion with the zero-error. This is the more advisable, since a 
reference of the zero-point in a completed barometer to any point of the scale is 
rendered difficult and uncertain by the circumstance, that it can only be viewed 
through the glass of the cistern, which, from its irregularity, may considerably affect 
its apparent position. It is therefore the practice to suspend the barometer to be 
examined beside the Standard ; to make a sufficient number of simultaneous observa- 
tions of the two instruments ; and to adopt the mean difference of their indications 
as a single constant correction for the combined effects of zero-error and capillary 
action. 
In many portable barometers, and in nearly all marine barometers, there is no 
means of adjusting the mercury in the cistern to a constant level : it becomes there- 
fore necessary to determine the correction for “ Capacity,” or the variation in the 
zero-point corresponding to different heights of the column of mercury. The amount 
of this correction may be determined during the construction of the instrument; or, 
by reducing in the required proportion the lengths of the divisions, it may be allowed 
for in graduating the scale; as has been done in the marine barometers made under 
the supervision of the Kew Committee by Mr. P. Adie of London. In order to test 
the accuracy of this correction, it is necessary to compare the barometer at two con- 
