1871.] H. F. Blanford— -The Calcutta Standard Barometer. 447 



by Professor Balfour Stewart, and in Ceylon, for instance, at 

 the observatories established two years ago by Captain Fyers, the 

 barometric registers are corrected to the readings of the Kew 

 Standard. 



The receipt, some months ago, of a number of very excellent stan- 

 dard barometers by Casella, which had been compared at the 

 India Store Department with a Standard, the error of which to 

 those of Kew and Greenwich had been previously ascertained, has 

 afforded me an opportunity of ascertaining indirectly the difference 

 of the Calcutta standard from these latter instruments, and thus 

 obtaining a correction which will render the barometric registers 

 of Northern India more rigorously comparable than heretofore 

 with those of Ceylon and other places. The comparison, as will be 

 seen, is very indirect, but it has I think been sufficiently extensive 

 to prevent any appreciable error arising from the cause, always 

 assuming that the India Store Department Standard is accurate- 

 ly corrected to those of Kew and Greenwich. 



The instruments which have afforded the means of this compari- 

 son are of the form figured in Messrs. Casella's catalogues, with 

 tubes of about 0*3 inch internal diameter. Eight of them have 

 been compared by myself with Newman's Standard, No. 94. They 

 were compared one at a time, placed side by side with the latter 

 instrument, and each was read simultaneously with the Newman, 

 nine (in two cases ten) times daring the diurnal period of rising, 

 and an equal number of times during that of falling pressure. 

 By this means the correction for capillarity, always more or less 

 uncertain, is eliminated. The readings of each instrument have 

 been reduced for temperature by those of its own attached thermo- 

 meter, so that the corrections obtained include those, if any, of the 

 thermometers. The total number of comparative readings is 146. 

 The results of this comparison are given in the following tab 



The error of Newman's barometer, No. 94, with the standard at 

 the Surveyor General's Office has been ascertained as follows. The 

 former instrument was compared by Babu Gopinath Sen before 

 I received it three years ago, and the mean error of 13 reduced 

 readings then determined to be — .0245. A series of levels, from 

 the ground floor of my house to that of the observatory, shewed 



