Page:Cyclopaedia, Chambers - Volume 1.djvu/979

 HEA

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HEA

riiminim the Morion imprefs'd by the paft Action of his Rays wherein Heat confifts, before he rifes again 5 but under the Pole, the long Abfence of the Sun for fix Months, wherein the Extremity of Cold does obtain, hath fo chill'd the Air, that it is, as it were, frozen, and cannot, before the Sun has got far towards it, be any Ways fenfible of his Prefence, his Beams being obftru&ed by thick Clouds, and perpetual Fogs and Mifts.

Add, that the differing Degrees of Heat and Cold in different Places, depend, in great Meafure, upon the Ac- cidents of Situation, with Regard to Mountains or "Valleys, and the Soil. —The firft greatly help to chill the Air by the Winds, which come over them, and which blow in Eddies through the Levels beyond. See Wind.

Mountains, fometimes, turning a Concave Side towards the Sun, have the Effect of a burning Mirror on the Subject plain j and the like Effecl is fometimes had from the Concave or Convex Parts of Clouds, either by Refraction or Reflection. And fome even take thefe to be furficient to kindle the Exhalations lodg'd in the Air, and produce Thunder, Lightning, &c See Mountain, Mirror, &c.

As to Soils : A ftony, fandy, or chalky Earth, 'tis known, reflects moft of the Rays into the Air again, and retains but few ; by which Means a confiderable Accefllon of Heat Is derived to the Air ; as, on the contrary, black ones abforb moft of the Rays, and return few into the Air, fo that the Ground is fo much the hotter. See Black- ness, Whiteness, &c.

This the Peafants, who inhabit the Morafs de Veenen t where Turf is dug, are very fenfible of; walking there but a little while, the Feet grow extremely hot, but the Face not at all : On the contrary, in a fandy Place, the Feet are fcarce warm when the Face is fcorch'd by the great Refleaion.

The following Table gives every tenth Degree of Lati- tude to the Equinoctial and Tropical Sun, by which an eftimate may be made of the intermediate Degrees.

Lat.

Sun in

Sun in

Sun in

Y. St.

©

Vf

20000

18341

18341

11

1 969 6

20290

15854

20

18797

"737

13161s 1

30

17321

22551

10124

40

15321

23048

5944

50

12855

22991

3798

60

1 0000

"773

1075

7°

(1840


 * 3543

000

80

3473

24*7 3

000

90

0000

25055

000 |

Whence are deducible the following Corollaries :

1°. That the Equinoctial Heat, when the Sun becomes vertical, is as twice the Square of the Radius : Which may be propofed as a Standard to be compared with, in all other Cafes.

2". That under the Equinoctial, the Heat is as the Sine of the Sun's Declination.

3 . That in the frigid Zones, when the Sun fets not, the Heat is as the Circumference of a Circle into the Sine of the Altitude at 6. And, confequently, that in the fame Latitude, thefe Aggregates of Warmth, are as the Sines of the Sun's Declination 5 and at the fame Declination of the Sun, they are as the Sines of the Latitudes into the Sines of the Declination.

4°. That the Equinoctial Days Heat, is every where as the Cofine of the Latitude.

5 . In all Places where the Sun fets, the Difference be- tween the Summer and Winter Heats, when the Declina- tions are contrary, is equal to a Circle into the Sine of the Altitude at 6, in the Summer parallel 5 and confe- quently thofe Differences are as the Sines of Latitude into, or multiplied by, the Sines of Declination.

6°. From the foregoing Table it appears, that the Tro- pical Sun, under the Equinoctial, has of all others the leaft Force : Under the Pole it is greater than any other Days Heat whatever, being to that of the Equinoctial as 5 to 4.

From the Table, and thefe Corollaries, a general Idea may be conceived of the Sum of all the Actions of the Sun in the whole Tear, and thus that Part of Heat, which arifeth fimply from the Prefence of the Sun, may be brought to a Geometrical Certainty. — The Heat of the Sun, for any fmall Portion of Time, is always as a Rectangle, contain'd under the Sine of the Angle of Incidence of the Ray, producing Heat at that Time.

Heat, is ufually divided, by the School Philofophers, into AQual and 'Potential.

Actual Heat, is that which we have been hitherto fpeaking of, and which is an Effect of real, elementary Fire.

'Potential Heat, is that which we find in Pepper, Wine, and certain chjmical Preparations, as Oil of Tur- pentine, Brandy, Quick-lime, ISc. See Potential.

The Terifaieticks account for the Heat of Quick-lime from an Antiperiflafis. See Antiperistasis.

The Epicureans, and other Corpufiulareans, attribute even potential Heat, to Atoms or Particles of Fire de- tain'd and lock'd up in the Bores of thofe Bodies, and remaining at reft therein ; which being excited to Action again by the Heat and Moiflure of the Mouth, or by the Effufion of cold Water, or the like Caufc, break their In- clofures, and difcover what they are.

This Doctrine is well illuftrated by M. Lemery the younger, in the Inftances of Quick-lime, Regulus of An- timony, Tin, (Sc. in the Calcination whereof he obferves, i°, That the Fire, which they imbibe in the Operation, makes a fenfible Addition to the Weight of the Body, amounting fometimes to one tenth of the whole ; and, that during this Imprifonment, it flill retains all the par- ticular Properties or Characters of Fire ; as appears hence, that when once fet at Liberty again, it has all the Effects of other Fire.

Thus, a ftony, or faline Body being calcined, and Water poured thereon, that Fluid is found fufficient, by its ex- ternal Impreffion, to break up the Cells, and let the Fire out ; and upon this the Water is rendred more or lefs warm, according to the Fire lodged therein. — - Hence, alfo, it is, that fome of thefe Bodies vifibly contain a deal of actual Fire ; and the flighrelt Occafion is capable of dif- engaging it : Upon applying them to the Skin, they burn and raife an Efchar, not unlike the Top of a live Coal.

To this it is objected, that the Particles of Fire aro only fuch in Virtue of the rapid Motion, wherewith they are agitated ; fo that to fuppofe them fixed in the Pores of a Body, is to divelt them at once of that, which conftituted them Fire ; and, confequently, to dis- qualify them for producing the Effects afcribed to them. — ■ To which M. Lemery anfwers, that tho' the rapid Motion of Fire do contribute very greatly to its Effects, yet the particular Figure of its Particles is to be confider'd withal. And tho' Fire mould be detained, and fixed in the Subftance of Bodies, yet why fhould it fare worfe than other Fluids in the fame Circumflances > Water, for In- flance, is a Fluid, whofe Fluidity depends, as already ob- ferved, on Fire, and confequently is lefs fluid than Fire 5 and yet every Day, Water inclofed in Bodies of all Sorts, without lofing its Fluidity, or any of rhe Properties that characterize it.

Add, that when Water is froze, the Motion of its Parts is, doubtlefs, difcontinued ; and yet the Figure of the Par- ticles remaining the fame, it is (ready to commence a Fluid, as before, upon the leaft warmth. See Water.

Laflly, tho' Salt be allowed to be the Matter of Taftes, and that it has certain Properties, arifing chiefly from the Figure of its Parts 3 yet it only aits when diffolved ; or, which amounts to the fame, when it fwims in a Fluid proper to keep its Parts in Motion : Yet it is not lefs Salt, or lefs the Matter of Tafte, when not in a State of Diffo- lution : To difpoil it of that Quality, the Figure of its Parts mutt be altcr'd. See Salt.

As to what may be further objected of the Impofllbi- lity of fixing fo fine, fubtle, penetrative, and active a Mat- ter as Fire, within the fpongeous Subftance of a grofs, porous Body; it will be of no great Weight, unlefs it can be proved, that the Pores of the Cells are bigger than the fame. — If it be infilled, again, that a Body which could find its Way into a folid Body, might get out again the fame Way ; and that as it only penetrated the Body, by Reafon its own Corpufcles were fmaller than the Pores, the fame Confideration muft let it out again : It is anfwer'd, that the Pores are not now in the fame Condition as be- fore ; the Fire, in calcining, open'd and dilated the Pores - which, upon the Fire's ceafing, muft clofe and contract again! Mem. de I' Acad. An. 171 j.

Mr. 'Boyle endeavours to fet afide this Account, and fiib-> ftitute a mechanical Property, viz. a peculiar Texture of Parts, in lieu of Fire. — Tho' a great Likenefs might be expected between the Particles of Fire adhering to the Quick-lime, and thofe of highly rectified Spirit of Wine ; yet he has not found that the Etiulion of the Spirit upon Quick-lime, produced any fenfible Heat, or vifible Diflolution of the Lime, tho' it feem'd to be greedily fuck'd in, as common Water would have been. And, furtheij he found, that if cold Water were poured on the fame Lime fo drenched, there would enfue no manifeil Heat ; nor did the Lump appear fwelled or broken, till fome Hours after 3 which feems to argue, that the Texture of the Lime admitted fome Par- ticles of the Spirit of Wine into fome of its Pores, which were either larger, or more fit, without admitting it into

the.