1878.] Comparison of Standard Barometers. 77 



from time to time indirect comparisons have been made by persons not 

 connected officially with either. 



Attention has recently been drawn to the snbject by the pnblication 

 of a memoir by Dr. H. Wild, Director of the Central Physical Observa- 

 tory, St. Petersburgh, entitled " Ueber normal-Barometer nnd ihrer 

 Vergleichnng," in which the anthor states (p. 100) that the standard 

 barometer of the Royal Observatory reads 0*465 m.m., or 0*018 inch 

 lower than that of the Kew Observatory, arriving at that conclnsion 

 by means of somewhat circnitons comparisons. Professor Mohn, of 

 Christiania, in 1876, writes also that he finds the difference Kew — 

 Greenwich to be + 0*49 m.m., or + 0*019 inch. 



In 1869 the late Colonel Strange made a comparison between the 

 two standards by means of a barometer which he had conveyed from 

 the one observatory to the other, the resnlts of which are stated by him 

 to be as follows : — 



By comparison No. 1 Greenwich lower than Kew 009 inch. 



„ 2 „ .„ 0*006 „ 



„ 3 „ „ 0*004 „ 



In consequence of these statements the Kew Committee made appli- 

 cation to the Astronomer Royal for permission to make a direct com- 

 parison between the two instruments, and received from him a courteous 

 consent to their proposition. 



Accordingly, having selected four barometers — Adie 655, Adie 657, 



C C 

 Negretti and Zambra — -, and Negretti and Zambra — — , all standards 

 4^9 4ol 



on the Fortin principle — as suitable instruments for the purpose, I 



made arrangements to convey them to Greenwich and back safely in a 



carriage. 



On April 23rd, 1877, Mr. Baker and I made a set of observations 

 of the two large Kew standards, constructed by the late Mr. Welsh ; 

 Newman 34 [the working standard of the Observatory], Adie 655 and 

 Adie 657, and subsequently of Newman 34, and the two Negretti and 

 Zambra standards. 



The latter we also compared again on the morning of the 24th, 

 before starting for Greenwich. 



Having carefully packed the instruments, accompanied by an assist- 

 ant, I went to the Royal Observatory, and there suspended the 

 barometers in the large room of the Magnetic Observatory, facing and 

 about eight feet distant from the standard barometer (Newman 64). 



After leaving the instruments about an hour, in order that they 

 might assume the correct temperature of the room, which did not 

 differ greatly from that of the external air at the time, we made a set 

 of twelve readings of all the four instruments and of the Greenwich 

 standard, myself and the assistant reading them alternately, the tern- 



