barometrical column stands at 18°. Indeed, the experiment is often
exhibited at our chemical lectures, of a flask containing a small
quantity of water, which, exhausted of air, is made to boil by the
ordinary heat of the hand.

Fahrenheit proposed to ascertain the height of mountains by this
principle, and a simple apparatus was contrived for the purpose, which
is now in successful use. The late Professor Forbes of Edinburgh,
whose untimely death the friends of science have had so much reason to
deplore, ascertained that the temperature of boiling water varied
arithmetically with the height, and at the rate of one degree of the
thermometric scale for every 549.05 feet. Multiplying the difference
of the boiling-point by this number of feet, we have the elevation.
The weight of the atmosphere, as indicated by the barometer, is also a
means for ascertaining the height of mountains or of plains; but
correction must be made for the effects of expansion or contraction,
and for capillarity, or the attraction between the mercury and the
glass tube, at least whenever great exactness is required. Tables for
the convenience of calculation are given in several scientific works,
and particularly in a paper of Professor Forbes, Ed. Trans. Vol. 15.
Briefly, however, we may state, that between 0° and 32°, 34
thousandths of an inch must be allowed for depression or contraction,
and between 32° and 52° 33 thousandths. The weight of the atmosphere
is not only affected by rarefaction, but by currents of air, which
give it a sudden density or rarity. Those who have ascended mountains
have experienced both these changes.

A common experiment to prove the weight of air is that of the
Magdeburg Hemispheres, a simple contrivance of Otto Guericke, a
merchant of that city. It is a part of every complete philosophical