680 
Kffects of contracted by the addition or subtraction of heat, it is. 
Heat. 
and thus mark the degree of heat. The Florentine 
by the Flo- academicians had the merit of considerably improving 
rentine aca- the apparatus of Santorio, by substituting alcohol: for 
demicians. air, and by confining it in a tube nearly similar to the 
Halley, one now in use. Halley is said to have been the first 
who employed mercury, and Newton made the very 
Newton, ij impr f forming a regular scale, b: 
important improvement of fo ig a reguiar , by 
Hooke. which the observations might be recorded and.compared 
together. It is, however, to Hooke that we are indebt- 
ed for what may be regarded as a still greater improve- 
ment, viz. the formation of fixed points in the scale, by 
which, at all times and in all cases, thermometrical eb- 
servations might be compared together, and be, as it 
were, all referred to one invariable standard. He per- 
ceived that water became solid always at the same tem- 
perature, and that, under the same atmospherical pres- 
sure, the fluid always becomes conve! into vapour 
at the same degree of heat. Hence it follows, that if 
we immerse a tube containing mercury first into freez- 
ing, and afterwards into boiling water, we obtain*two 
stationary and invariable degrees or points in the scale, 
which will bear the same relation to the temperature of 
other bodies, whatever be the size of the instrument, or 
under whatever circumstances the experiment is. per- 
formed. Having ascertained these points, the numbers 
which are affixed to them, or the number of degrees 
into which we divide the interval between. them, is en- 
tirely arbitrary, and comparatively unimportant. 
Different The thermometric scale in general use in this coun- 
thermome- try was formed by Fahrenheit, in which the cypher or 
tric scales. ero is supposed to be indicated by a mixture of com- 
mon salt and pounded ice. ‘The freezing point of wa- 
ter is fixed at 32°, and the boiling point at 212°, so 
that the interval between the freezing and boiling of 
water comprehends 180°. In France, before the Re- 
volution, the thermometer of Reaumur was generally 
employed. Here the freezing of water is the zero, or 
the commencement of the scale, and boiling water is 
assumed as the 80th degree. Since the Revolution, the 
centigrade scale is employed, which had. been previous- 
ly adopted by Celsius, a Swedish philosopher. Here 
freezing water is. marked 0°, and boiling water 100%. 
Perhaps none of them are without considerable. objec- 
tions. Fahrenheit and Reaumutr’s seale are completely 
arbitrary; while the centigrade ‘has its S too 
large for many purposes, and it has also the defeet of 
beginning too high, so that we have very frequent oc- 
casion. to. employ the terms. + and —, which may lead 
to confusion. Of the-three, that of Fahrenheit is cer- 
tainly the best; both because it is, ina great measure, 
free from this last objection, and because the degrees are 
not too large. Nor is it of much consequence that'the 
commencement and termination. are so entirely: arbitra- 
ry; for when we are once. accustomed to it, the mind 
refers to it as a term of. comparison, without ever consix 
dering the reason for fixing upon the numbers that are 
employed. 
‘The fluids employed in the formation of thermome- 
ters are either mercury or aleohoh Where we wish to 
ascertain very low temperatures, the latter is. preferable, 
because it cannot be frozen but. by the most intense 
cold, see Conn, vol. vi. p. 795. whereas mercury: be- 
comes solid at —39°, But for every other purpose, 
mercury has a decided advantage. | Its expansions from 
the same additions of heat are more uniform; it. is 
also. more sensible to slight variations of temperature ; 
and its boiling point is so elevated, that it possesses a 
range in the upper partof the scale, which is. applica 
Improved 
HEAT. 
obvious that. the fluid would. sink or rise'in the tube, . 
: 
: 
e temperature of a mixture of two 
portions, one at the freezing, and_the other at the boil- 
ing point, was 24° Jess than the mean. The real tem- 
perature in this case is 122°, but, according to the com- 
mon graduation, the thermometer stands at 1244°, (Re- 
cherches, t. i. p. 311.) Hence, in order to obtain a pre- 
ry em 
cludes most cases that usually fall under. our observa. ‘tes — 
tion, yet we have not unfrequently oceasion to mea- 
sure much higher degrees of heat. Newton attempted Newton’s 
to accomplish this point, without any apparatus directs theorem. — 
ly adapted to the purpose, by a species of computation, if 
He ascertained the rate at which bodies cool, compas  . 
red to the heat. of the surrounding medium, at tempe- 
ratures within the reach of the thermometer, and he 
then extended the same ratio to: the hi degrees of 
heat.. By this very ingenious method, he discovered 
the temperature of red hot iron, and. the melting point 
of mapy of the metals, with a considerable degree of 
precision. Since his time, different instruments, call- 
ed. pyrometers, have been invented, of which the most 
valuable, and that which has. come into the most ge- ’ 
neral use, was invented by Wed, In the course Wedves 
of his: riments on the manufacture of earthen ware, yooq°s 
he: perceived that: the differ _ Peps oa » called rometer, 
clays, into the c ition of which a quanti 
“lewis pace ep ey heen = property of a 
tracting by exposure to a great heat, and likewise that 
this contraction was in proportion to the degree of heat 
bs a When the clay has once contracted, its 
bulk remains permanently diminished, so that by ap« 
plying it to a guage, previously graduated for the pur- 
pose, we can ascertain to what degree of heat the 
piece of clay in question has been exposed. The guage 
by which the contraction of the-clay is ascertained, con- 
sists of two straight pieces: of . brass, fixed. on a. brass 4 
plate in such a manner that they are nearertogetherat one an 
end than at the other; and thus the more the clay has 
been contracted, the farther will it slip between the 
brass bars. Wedgewood then proceeded to examine 
the relation between “his pyrometer and the common 
mercurial thermometer, and he, determined that the, 
