THERMOMETRY. 
491 
9° Fahrenheit, 7J° De ITIsle, 5° Centigrade, and 4° Reaumur, 
are severally equal. The conversion of degrees upon one scale 
into those of another is easily effected by the aid of certain 
simple formulae or tables, which will be found in most books 
treating of the subject. Practically, the conversions which 
most frequently occur are between Fahrenheit and the centi- 
grade ; the former may be regarded as the established scale ; 
the latter the innovating one, pushed forward by the decimal 
gentlemen, now seeking to indoctrinate the English mind with 
their hobbies. 
It has been found by experiment that mercury when raised 
from 32° to 212° increases in volume by T f T ths of its value at 
32°. Thus 111 cubic inches of mercury at 32° will, if raised 
to a temperature of 212°, become 113 cubic inches. From 
this may be deduced the increment of volume which mercury 
receives for each degree of temperature. For since the increase 
of volume corresponding to an elevation of 180° is T f T ths of 
its volume at 32°, we shall find the increment of volume cor- 
responding to 1 D by dividing T f T ths by 180, which is 
It follows, therefore, that for each degree of temperature by 
which the mercury is raised it will receive an increment of 
volume amounting to g-^oth its volume at 32°. It follows 
also that the weight of the mercury which fills that portion of 
a thermometer tube representing 1° of temperature will be the 
g-Jgo-th of the total weight contained in the tube. We here 
assume that equal increments of heat produce equal dilata- 
tions of the mercury in the tube, and it has been found by 
careful experiments that such is actually the case between the 
freezing and boiling points, but that at extreme temperatures 
the mercury does not uniformly expand and contract. Since, 
however, the meteorologist is never called upon to consider 
extreme temperatures, we may pass over further allusion to 
this matter. 
It may be well here to mention in a summary form the 
reasons why mercury is so generally adopted for thermometric 
purposes. It is highly sensitive to changes of temperature, 
contracting and dilating quickly under the influence of varying* 
temperatures ; it freezes at a low and boils at a high tempera- 
ture, and at temperatures which are not near these extremes, 
its expansive and contractive powers are very uniform. It does 
not vapourize and become broken up into small particles in so 
confined a space, nor does it change its bulk by adhering to 
the sides of the glass. These are the reasons which have led 
to its so extensive use. The freezing-point of mercury being 
—40° and its boiling + 600°, a mercurial thermometer will 
afford correct indications through a very long range of tem- 
peratures. 
