444 CONSTRUCTION OF A STANDARD BAROMETER. 





are not rapid, so that no sensible error arises from assuming the tem- 

 perature of the cathetometer to be the same as that of the mercury. 

 The cistern of the standard barometer is 33.9 feet above the mean 

 level of the sea, being 9.1 feet above the ordnance bench-mark on 

 the northeast corner of the observatory, whose elevation is stated by 

 Lieutenant Colonel James to be 24.83 feet. 



Observations of this barometer being too troublesome when an 

 extensive series is required, a standard by Newman, ('No. 34,) having 

 a tube of 0.55 inch, which has been recentty compared with the great 

 Kew standard, is employed for ordinary use, its index correction 

 (which, inclusive of capillary action, is — -|- 0.003 inch) being first 

 applied to the observed readings. 



Comparisons, by means of two portable barometers by Adie, Lon- 

 don, were made during last summer between the Kew standard and 

 that of the observatory at 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. — Verification op 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 inspection, the only source of 

 error in such instruments is to be looked for in the scale. The gradu- 

 ation of the scales of all carefully made barometers is performed by 

 means of a dividing engine, and it 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 eleva- 

 tions, 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, how- 

 ever, 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 capil- 

 lary action of the tube is also supposed to be constant for the same 

 barometer, and as it is seldom possible to determine its true amount, 

 it is better to consider it in connexion with the zero error. This is 

 the more advisable, since a reference to 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 baro-. 

 meter to be examined beside the standard, to make a sufficient num- 

 ber of simultaneous observations of the two instruments, and to adopt 

 the mean difference of their indications as a single constant correc- 

 tion 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 to a constant level. It 



