recur, it is of great use to have means of abbreviation, I have given a
table in the appendix for reducing lines and fractions of lines into
decimal fractions of the inch.
In experiments performed in the water-apparatus, we must make similar
corrections to procure rigorously exact results, by taking into account,
and making allowances for the difference of height of the water within
the jar above the surface of the water in the cistern. But, as the
pressure of the atmosphere is expressed in inches and lines of the
mercurial barometer, and, as homogeneous quantities only can be
calculated together, we must reduce the observed inches and lines of
water into correspondent heights of the mercury. I have given a table in
the appendix for this conversion, upon the supposition that mercury is
13.5681 times heavier than water.
SECT. VI.
_Of Corrections relative to the Degrees of the Thermometer._
In ascertaining the weight of gasses, besides reducing them to a mean of
barometrical pressure, as directed in the preceding section, we must
likewise reduce them to a standard thermometrical temperature; because,
all elastic fluids being expanded by heat, and condensed by cold, their
weight in any determinate volume is thereby liable to considerable
alterations. As the temperature of 10 (54.5 ) is a medium between the
heat of summer and the cold of winter, being the temperature of
subterraneous places, and that which is most easily approached to at all
seasons, I have chosen that degree as a mean to which I reduce air or
gas in this species of calculation.
Mr de Luc found that atmospheric air was increased 1/215 part of its
bulk, by each degree of a mercurial thermometer, divided into 81
degrees, between the freezing and boiling points; this gives 1/211 part
for each degree of Reaumur's thermometer, which is divided into 80
degrees between these two points. The experiments of Mr Monge seem to
make this dilatation less for hydrogen gas, which he thinks is only
dilated 1/180. We have not any exact experiments hitherto published
respecting the ratio of dilatation of the other gasses; but, from the
trials which have been made, their dilatation seems to differ little
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