399 
on the rate of chronometers. 
transporting a time-keeper from London to Geneva, would 
have been observed. 
The sudden changes to which the density of the atmos- 
phere is sometimes liable in this climate, renders it necessary, 
therefore, that a correction should be applied to the rate of 
a chronometer, proportional to the alteration of density ; the 
correction partaking in some cases of a positive character, 
and in others of a negative. A similar correction must like- 
wise be necessary when a traveller ascends to any consider- 
able elevation above the sea; for example, to Geneva, to 
the plains of the Castiles, or to the table land of Mexico. 
The value of the correction will be different for different 
time-keepers, and in all cases must be determined by pre- 
vious experiment. 
The changes here alluded to can influence chronometers 
only beyond the tropics, since between them, it is known 
that the fluctuations of the barometer do not much exceed 
a quarter of an inch ; but in the arctic regions, where the 
causes which promote alterations of atmospheric density are 
the greatest, the effect on the time-keeper must be the 
greatest also. 
In proportion however as we ascend above the level of 
the sea, the uncertain changes of the barometer are known 
to approximate to uniformity; and therefore at higher 
elevations, the same chronometer would preserve a greater 
regularity of rate than in the lower regions of the air. 
It becomes now an interesting question to determine, if the 
alterations of rate displayed by the different chronometers, 
under the various circumstances in which they have been placed 
in the preceding experiments, is immediately acquired the 
