1878.] 



Comparison of Standard Barometers. 



79 



On the 16th the Greenwich observers made eight sets of readings, 

 on the 17th seven sets, and on the morning of the 18th two sets. 



Later on the same day Messrs. Baker and Foster made twelve sets, 

 carrying the barometers again back to Kew, where, on the 19th, twelve 

 sets of observations were finally taken. 



On the 23rd the Kew barometer (Newman 34) was again compared 

 with the two Normal barometers of the Observatory, and its correction 

 found to be — O007 inch, the same as determined on April 23rd. 



The readings at Greenwich on the 17th May were taken at a time 

 when the pressure was rapidly falling, and a storm passing over the 

 Observatory ; and on the 18th its effect had not completely passed off. 

 the wind being high and the temperature changing somewhat quickly . 



The observations made on these two days were discordant from 

 these causes, and have therefore been left out of the comparison, and 

 the result of the third series of observations gives the difference 

 (Greenwich — Kew) + 0"0014 inch. 



We then have the following determinations : — 



Mean difference from 1st series of 128 comparisons + 0'0016 inch. 



„ 2nd „ 144 „ + 0-0007 „ 

 „ 3rd „ 72 „ +0-0014 „ 



Final mean of 344 „ + 0'0012 



This value is unchanged if we weight the determinations propor- 

 tionally. 



A correction of — 0'006 inch has been applied throughout to the 

 readings of the Greenwich standard ; this correction having been made 

 by the Astronomer Royal to all the readings of that instrument since 

 August 30th, 1866 (when the long sliding rod, after removal, was re- 

 placed), to make them accord with the old Royal Society standard's 

 indications. 



Throughout the observations the temperatures of all the portable 

 barometers were observed by means of thermometers fixed to the 

 middle of their brass tubes, having the bulbs in immediate contact 

 with the mercurial column. 



The errors of these thermometers were all carefully determined at 

 Kew, and every reading has been corrected to the Kew standard. 



The Greenwich standard barometer is provided with two thermo- 

 meters — a large one with a bulb approximately of the same diameter 

 as the barometrical column, and placed about half way up, but at a 

 distance of 1*25 inches in front of it — and a small thermometer not 

 divided on its stem, whose bulb is immersed in the cistern of the 

 barometer itself. 



The readings of the latter instrument do not admit of great exacti- 



