210 Observations to determine the amount of 
The observations by Lieutenant Ross, in Tables XIII. to 
XV. inclusive, were obtained with a small variation transit 
instrument as an upper telescope, and those in Tables XVI. 
and XVII. by a pocket telescope below ; both being fixed to 
a cask filled with sand. None of the instruments used by 
either of the three observers were removed, till after the 
completion of the whole series of observations. 
The hour angle by which the true altitude of the setting 
star was determined, was obtained by taking its right ascen- 
sion from that of the meridian, at the time of observation, as 
found by transits of well known stars, which took place 
within three quarters of an hour of the other star’s setting, 
thus rendering the observations as independent as possible of 
any want of uniformity in the rates of the pocket chrono- 
meters employed by the observers. The transits were taken 
exclusively by Lieutenant Foster, and comparisons with the 
chronometer he employed, were taken by the other observers 
about the time of transit, in order to deduce their horary 
angles, contained in the respective Tables. The position of 
the transit instrument was rigidly verified by the transits of 
high and low stars in their passages across the meridian, as 
well as by a constant reference to a meridian mark, and by 
the most minute attention to the level. The heights of the 
barometer, and of the thermometer, suspended with its bulb 
on the same level with the observers in the open air, were 
taken at the time of every observation. The registered 
height of the barometer, however, in the Tables, has been 
corrected for instrumental errors, and brought up to a certain 
temperature, which is specified at the head of each of the 
columns containing it. 
