480 
H E A T. 
room was perfedtly fteady, and at the temperature of 15® 
centigrade. I noticed carefully die progress of the ball 
■in cooling : from the ft'ation of 35 0 , till the internal ther¬ 
mometer funk to the middle point, or 25 0 , the time 
elapfed was 156 minutes. I next painted the furface ot 
the ball with a coat of lamp-black, and, again filling it 
with warm water, fcrupuioufly repeated the experiment. 
The fame effect was now produced, or one-half of the 
heat expended, inthefpaceof only eighty-one minutes.” 
From this experiment, it is obvjous that the addition 
of a coat of pigment accelerates the cooling of the me¬ 
tallic ve.flel. But another queftion arifes, are bodies 
then cooled folely.by the pulfatory power with which 
they emit heat ? If they are, the metallie body ought 
4o cool eight times more flowly (fince S nearly = ——) 
.than the painted one : but we may eafily infer from the 
experiment that this is not the cafe ; for, fuppofing the 
deciement of heat to vary in the compound ratio of the 
temperature and increment of the time, d h CX .—d 1 
.{h heat, t the time) ; and to obtain an equation, (up- 
pofe the lofs of heat in one minute at the initial adt of 
cooling to be — (tf the whole heat at firfi) ; then, in x 
minutes, were the procefs of cooling uniform, dhz=. h. 
dt. — : confequently, — — hyp. log. — (if M the 
1 H ' 
.modulus be put = *43429448, &c.) jlog.—; hence 
■x= t X M. - 
juft quoted, : 
Let, 1 
w, h, as in the experiment 
b„t, 
log. 2 3OIO3O 
156, for the velfel coated with pigment = 81 ; hence x 
in the firft cafe = 
156. 
fecond cafe x = 81 X 
301030 
434294_ 
301030 
: 223 nearly ; in the 
! 17 nearly ; or, in other 
nifter when oppofed to the refledlor, and no effedt. was 
produced on the. differential thermometer ; that is, the 
afcended liquor remained at the fame'height. To de-' 
termine the firft part of the queftion, heated balls were 
expofed to the action of the wind, and the greater the 
wind, the greater was the rate of cooling. It is plain, 
therefore, that the renewal of the contact of frelh air 
accelerates the cooling of a body ; and it may eafily be 
.■Ihown, ■ that the effebl of fuch a renewal is the fame, 
whatever be the nature of the heated furface/ 
As there is no doubt but the repeated contadt of air, 
without impairing the pulfatory energy, accelerates the 
procefs of cooling ; it becomes important to know the 
exadt efTeft. produced by a dream of air. From' expe¬ 
riments very ingemoufly inftituted, Mr. Leflie deter¬ 
mined that the refrigerant power of a ftream of air va¬ 
ries as its velocity. Hence, as, in the aft of cooling, 
motion rnuft be produced in the ambient medium, the 
.rate of cooling will depend on the celerity of thatmo- 
. tion; or on the celerity with which the contadl of the 
heated furface with frelh particles of air is renewed. 
The explanation of the additional caufes of refrigera¬ 
tion, are next given by this ingenious author. He fup-. 
pofes that a flow receftion of heated particles takes place, 
in lines perpendicular to the heated furface, and that 
the velocity of receftion varies as the excefs of tempera¬ 
ture : for, fince the fquare of the velocity varies’as the 
fpace multiplied into the force, and the force is mea¬ 
sured by the fpace through which the heated particles 
recede, (veff) 2 a (fpace) 2 or (force) 2 , and veP CX ex- 
panfive force, or as the excefs of temperature. Thefe 
heated particles, in receding, communicate their, heat 
to the f'urrounding fluid ; their place is fupplied by 
frelh particles ; and thus, by the i;enewal of .contadt, the 
refrigerating effedt is accelerated. On this intricate fub- 
jedt Mr. Lellie’s obl'ervations are new and full of inte- 
" ' air or gafeous fluid which 
receive that fame meafure of 
heat, and a correfponding increafe of elafticity. It con¬ 
fequently dilates with a force proportional to the fpace 
through which it recedes, or to the elevation of tempe¬ 
rature Which it has aifumed. But the fquare of the ac¬ 
quired velocity, as we formerly remarked, is compound¬ 
ed of the fpace and the adtuating force : in the prefent 
words, the metal furface muft have loft every minute cafe, it is, therefore, as the fquare of either of thefe ele- 
,the 225th part of its heat, and the painted the 117th part, nients, or as the fquare of the degree of heat which is 
Confequently, vefiels are cooled not l’olely by their pul- abforbed. The velocity of propulfion is hence propor- 
fatory power of emitting heat; for, if they did cool tional Amply to the excefs of temperature. The time 
folely by that power, when th.e painted furface loft the of adtion is always evidently the fame, becaufe, if the 
117th part of its heat, the metal one ought to lofe only fpace be enlarged, the rate of dilatation is like wife in- 
the 936th part. creafed ; and hence, from every exciting point of the 
There are unqueftionably other caufes, befides that hot furface, a (lender continued ftream of air is emitted 
,cf pulfatory energy, by which bodies lofe their heat, perpendicularly, whofe velocity is proportioned 
If a hot body be immerfed in water, it emits no heat by 
pulfation : but it rapidly cools ; and with the fame ra¬ 
pidity whether its lides be metallic, or' covered with 
pigment. If the air, then, were deprived of its elafti- 
rify, the procefs of cooling in it would be fimilar to that 
meafure of heat inceffantly communicated. When the 
procefs is inverted, and the furface affedted is colder 
than the furrounding atmofphere, the contiguous por¬ 
tions futfer contradtion and a diminution of their elaf¬ 
ticity, which occalions a gentle perpendicular flo 
..hich takes place in water; and tiiough the elafticity redted towards the fource, and produdtive of a fimilar, 
of the air may perhaps modify or interrupt this procefs, though an oppofite, effedt. Thus the difcharge of heat 
it cannot prevent it. Part of the fluid circumambient from a body is materially promoted by the foft propel- 
the heated body muft become fpecifically lighter, and lent motion excited continually at its furface. This ef- 
afcend. Again, particles of the fluid muft come in con- flux extends to a very fliort diftance, before it fpends 
iadt with the heated furface, and, from an increafed ex- its force and lofes itfelf in the atmofphere; yet it eqdal- 
panfive force, recede in curves convex to horizontal ly produces the refrigerating effedt, by quickening the 
lines drawn from the refpedtive points of contadt. On circulation and frefh -contact of the ambient medium, 
both thefe accounts, motion muft be produced in the Though it confpires with pu.lfation to accelerate the 
ambient medium, and the heated furface muft be vifited difperiion of heat, it differs effentially in it* charadter 
by frelh portions of fluid : a queftion then arifes, whe- from that fpecies of energy. Pulfation is the fame at 
ther, and in what ratio, the renewal of frelh air accele- all degrees of heat, and its intenfity depends merely on 
rates the cooling of bodies ? and, whether the renewal the nature of the bounding furface : but the perpendicu- 
of the contadt of frelh air impairs the pulfatory power lar flow is more vigorous in proportion to the excefs of 
of difcharging heat ? To determine the latter part of temperature, and has no relation whatever to the quali- 
the queftion, the author, by means of a pair of bellows, ties, phyfical or mechanical, of that furface. It was 
impelled a current of air along the heated fide of the ca- Ihown that only a very few particles diffeminated in the 
conti>- 
