100 



RECORD AND REDUCTION 



of the mercurial barometer is, therefore, — 0.22 inches. This quantity, strictly 

 speaking, is composed of two parts ; the first, the true index error of the aneroid, 

 and the second, the specific difference of the two instruments in different latitudes, 

 the mercurial barometer (weighing a mass of mercury against a mass of air) being 

 independent of a change of gravity, whereas the aneroid barometer is sensible to 

 any increase of gravity as we proceed to the northern high latitudes. Within the 

 limits of latitudes G6°.0 N. and 75°. 3 N. this variation amounts to 0.014 inches; 

 and its greatest difference from the mean, say in latitude 72°. N., is, therefore, 

 + 0.008 inches. This quantity being smaller than the uncertainty of the results 

 by the aneroid, I have considered it as a correction that can safely be neglected. 

 The formula b — b ir , (1 — 0.0026 cos 2 q>) shows the variation for any latitude <p. 

 North of latitude 45° the aneroid gives the higher readings. 



Resulting mean ^-hourly and mean monthly readings of the mercurial barometer in 

 the months of September, 1857, and February and April, 1858. — The results for 

 these months, given above, require a small correction to refer them from part of 

 the month to the whole month ; this was obtained by means of the known aneroid 

 readings for the interval when the mercurial barometer was not read, the index 

 correction — in .22 having first been applied. We find — 



Referred mean readings of the mercurial barometer for the full months of Sep- 

 tember, February, and April, of the first year : — 



The following comparisons were made for the purpose of ascertaining how near 

 the mean of 6 and 12 observations a day approximate to the true daily mean as 

 derived from hourly observations. The following mean hourly readings, taken for 

 15 days between January G and January 22, 1858, are taken from the record; also 

 the means for 7 days in January, 18-39, and for 15 days in July, 1859. (Of these 

 observations I find only the results recorded.) 



