236 New Form of Mountain or other Barometer. 
observation of coincidences therefore to be much more precise and 
agreeable. 
The reading is not nearer than to 0.01 of inches; which is quite 
refined enough for any purpose to which I may expect its appli- 
cation, and indeed I might say for every purpose, except the deter- 
mination of some atmospheric constants, when all recognized cor- 
rections would have to find their place. Whenever in such case 
or any other, all those corrections are employed, it will be appro- 
priate to read the measurement closer and closer. At present 
there seems to be no need. 
For instance, in the climate of Baltimore, for at least half the 
year, a change of 1° Fahr. in the dew-point implies an elevation 
or depression of the barometer of more than 0.016: such a change 
is very common, in the same locality, within the hour—such a 
difference highly probable, at the same moment, between places 
not very remote even from one another. Yet in the multitude 
of barometers read to thousandths, who reads the hygrometer ? 
So, in the application of the barometer to other than strictly 
meteorological purposes, the correction of a zenith distance for re- 
fraction when the angle is as great as 85°, is upon 0.01 of an inch 
of mercury only 0.2 of a second—a quantity to be sure general- 
ly admitted in calculation, but not materially affecting the most 
of astronomical or geodetic results. In zenith distances not so 
great, more usual, and more reliable, the amount of correction is 
still less: an altitude of 45° varies for 1, in the barometer less 
than two hundredths of a second, a quantity in all ordinary cal- 
culations to be safely neglected. Similar considerations apply to 
this instrument, when rendered portable for the determination of 
heights. If the thermometer and hygrometer remain the same, 
a variation of ;,1,; in the barometric column would correspond 
to a difference of level of little more than 10 inches; a space far 
within what any barometric observation has yet pretended. to an- 
swer for. 
The correction, too, arising from the specific gravity of the mer- 
cury, a particular rarely registered with the measurements, en- 
closes within no narrow margin the apparent accuracy of a read- 
ing to thousandths of inches. If we take 13.6 as a mean specific 
gravity, giving with a certain temperature and dryness a stand 
of 30 inches, a specific gravity of 13.601 would give an equiva- 
lent stand under the same circumstances of only 29.9978 inches. 
