, and an egg cannot be cooked there in an open vessel.
At Potosi the boiling-point is still lower, being 188 deg., and the
barometrical column stands at 18 deg.. 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.
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