CAL

CAL

( IM )

difference of cooling will be very linking. Inthe former cafe il the water cools from 188° toi8o°, by fimple expofure for five minutes, itmay be lunk in the latter cafe from 188 0 to 170°by a ftrong blaft for the fame time. The com-parifon will be better if inftead of equal times,equal Ioffes of heat are cbferved, and then thecomparative rate of cooling will be inverfely asthe times. Now there is no reafon to doubtthat the radiation from the hot furface is thefame in both circumftances, and therefore thedifference mull be owing to the fttperior quan-tity of caloric conveyed away by the rapidmotion of the air.

We find too that air has no cooling or heatingeffect on a dry furface, unlefs it be colder orhotter than the fubftance on which it plays.Thus if the bulb of a thermometer of the tem-perature of the furrounding air be wiped per-feftly dry, and the ftream from a bellows bedirected on it, no rife or fall of the mercuryenfues : but on the other hand a ftream of warmair diredted on a piece of ice will melt it muchfalter than fimple expofure to an equally warmatmofphere. In all thefe cafes the fubjedt ofexperiment is fuppofed to be dry , for if moif-ture be prefent-, the encreafed evaporation pro-duced by an augmented velocity of air will atall times abftradt a large portion of heat, as willbe further explained.

Mr. Leflie’s experiments on this fubjedt arecurious and precife. A thin hollow globe ofbright tinned iron, drawn into a fine neckthrough which a thermometer was inferted, wasfilled with hot water, and fuffered to cool to acertain point without difturbancc in a cloferoom without a fire, the temperature of whichwas perfectly fteady. The fame experimentwas repeated with no other difference than thatthe outfide of the veffel was covered with blackpaint, the effedt of which (as mentioned underthe head of radiant caloric) would be to encreafe,to about eight times, the energy of the caloricradiated from the furface, the fame being afcer-tained by the effedt of the reflection of radiatedcaloric from a diftant concave tin reflector.The bright ball actually cooled to a given pointin 156 minutes, whilft the painted ball cooled

to the fame point in only 81 minutes. Thedifference between thefe two numbers is fuffi-ciently ftriking, but very far fhort of whatwould have happened had the lofs of heat beenoccafioned by mere radiation, fincq the effect 0 fthis latter is eight times more in the paintedthan the metallic furface, whereas in this cafeit is only about double. Therefore when aheated fubftance is expofed to a free atmofphere,though as much at reft as a clofe room and anequal temperature will enfure, a large portionof the cooling effedt is to be attributed to theconducting power of the furrounding air andthe motion excited in it by the heated bodyitfelf, as well as the radial ing power.

An encreafe in the motion of the air ftill fur-ther illuftrates this. The fame bright tin ball,and another fimilar ball, painted, were expofedin the evening (when the heating effedt of theatmofpherical refledtion would be fcarcely feltmore on the black than the metallic furface) tothe adtion of a gentle wind out of doors underperfectly fimilar circumftances. The fame lofsof heat as in the former experiment was nowproduced in the bright ball in 44 minutes, andin the painted ball in 35'. A pretty ftrongbreeze in another experiment reduced the refpec-tive times to 23' and 20-$'. A vehement windbrought them ftill further down to g 4 / and 9'.Thefe comparative experiments, and the in-ferences deduced from them by calculating theentire range of cooling on the fuppofition of an,uniformly equal lofs of heat, T point out twoimportant circumftances, the one, that theadtual rate of cooling is molt materially affedledby the motion of the air; and the other, thatthe more rapid the motion of the air is, thelefs is the comparative effedt of the nature ofthe heated furface : or in other words, that theproper condudting power of air adts apparentlywith equal energy on all furfaces, and thereforeby abftradting heat fo powerfully it uniformlydiminifhes, and finally renders almoft infenfiblethe peculiar differences of the radiating procefs.The above experiments will be better feen in atabular form.

An equal lofs of heat was experienced by the

minutes. minutes.

bright ball in 156 and by the painted ball in 81 (which is as 1000 : 5x9) hi a clofe room.

•-■—4+ -- 35 (-1000 : 795) in a gentle breeze.

-23 --203- (-— 1000 : 880) in a ftrong breeze.

——-9) — -- -- 9 (----—1000 :947) in a violent wind.

Leflie, p, 268.