Mr. Harvey on the effects of the density of air, &c. 373 
conditions, the chronometers were placed beneath the capa- 
cious receiver of a large double barreled air pump, the pres- 
sure being indicated by an excellent mercurial guage. 
To prevent any irregular effects from the unequal action 
of terrestrial magnetism, the position of each chronometer, 
with respect to the meridian, was preserved constant during 
the whole course of experiments.* 
The first chronometer selected was an eight day one, of 
the box kind ; and which, for the purpose of farther refer- 
ence, I shall distinguish by the letter A. Its rate for ten 
days previous to the experiments was steady and uniform, 
amounting to — s".i, the mean pressure of the atmosphere 
being 30.1 inches; but when placed beneath the receiver of 
the air pump, under a constant pressure of 20 inches of the 
mercurial column, the mean of four days observation gave 
an equally steady rate of — i".3, the chronometer having 
gained i".8, by diminishing the density of the air in the ratio 
of 3 to 2. By farther exhausting the air, so as to make its 
density correspond with ten inches of the column of quick- 
silver, the mean of a like number of days gave a remark- 
ably steady rate of -J- i".7, being an increase of s".o on the 
former rate ; and by continuing the exhaustion until the 
mercurial column sunk to an inch, the average of the same 
number of days produced a rate of -{- 6 ". 6 , being a farther 
increase of 4". 9. Thus, an alteration of + 9 " .7 in the rate of 
the chronometer was prodused by diminishing the density of 
the air in the ratio of 30 to 1 ; and on removing it from the 
* That chronometers, having their balances magnetic, experience alterations 
from being placed in different positions with respect to the magnetic meridian, is no 
longer doubtful. 
