Page:Cyclopaedia, Chambers - Supplement, Volume 1.djvu/635

 EAR

EAR

magnetic. effluvia moving from one pole. to. the qther. Se« Works abr. vol. I. p. 285, 290..

Dr. Knight alfo thinks, the Earth may be considered as a great loadftone, whole magnetical parts are difpofed Id a very irregular manner ; and that the fouth pole of the Earth is analogous to the north pole in magnets, that is, the pole by which the magnetical ftream enters. SeeMAG.NET. He obferves, that all the phaemomena. attending, the direction of the needle, in different parts of the Earthy in great "mea- fure correfpond with what happens to a needle, when placed upon a large terella ; if we make allowances for the different difpofitions of the magnetical parts, with refpedt to each other, and confider the fouth pole of the Earth to be a north pole, with regard to magnetifm. The Earth might become magnetical Dy the iron ores it contains, for all iron ores are capable of magnetifm. It is true, the globe might notwithstanding have remained unmagnetical, unlefs fome caufe had exifted capable of making that repellent matter pro- ducing magnetifm move in a ftream thro' the Earth. See Magnet.

Now the Doctor thinks that fuch a caufe does exift. For if the Earth revolves round the fun in an ellipfis, and the fouth pole of the Earth is directed towards the fun, at the time of its defcent towards it, a ftream of repellent matter will thereby be made to enter at the fouth pole, and come out at the north. And he fuggefts, that the Earth's being in its peri- helion in winter, may be one reafon why magnetifm is ftronger in this feafon than in fummer.

The caufe here afligned for the Earth's magnetifm muff continue, and perhaps improve it from year to year. Hence the doctor thinks it probable, that the Earth's magnetifm has been improving ever fince the creation, and that this may be one reafon why the ufe of the compafs was not difcovered fboner. See Dr. Knight's Attempt to demon/Irate, that all the Phenomena in Nature may be explained, by Attraction and Repulfion. Prop. 87. feq. Motion of the Earth — The annual motion of the Earth about the fun is now generally admitted by aftronomcrs. The principal argument againft this annual motion of the Earth, is

the want of an annual parallax in the fixed ftars. For let TA( reprefent the Earth's orbit about the fun S, T X the axis of the earth, and A x parallel to T x, will reprefent the pofition of the fame axis at the oppofite point r. Suppofe TX to be directed towards the ftar P ; and it is manifeft, that the axis of the Earth will not be directed to the fame ftar, when it comes to the fituation t x, but will contain an angle xtp-with the line rP, joining the Earth and ftar, equal to the angle fPT, under which the Diameter Tt of the Earth's orbit appears to a fpectator, viewed from the ftar P. It might be expected therefore, that by obferving the fixed ftar P, from the different parts of the Earth's orbit, we might judge of the angle TP /, and confequently of the proportion of T P, the diftance of the ftar to T r, the dia- meter of the Earth's orbit, or double diftance of the fun. Yet it is certain that aftronomers have not hitherto been able to difcover any difference in the apparent fituation of the fixed ftars, with refpedt of the axis of the Earth, or to one another, that can arife from the motion of the Earth : Tho', fince the reftoration of the Pythagorean doctrine of the Earth's motion, they have taken great pains to examine the matter.

The feeming motion of the pole ftar, obferved by Mr. Flam- ftead, was by fome miftaken for a proof of its annual pa- rallax ; but this phenomenon has fince been accounted for in the moft ingenious manner by Mr. Bradley, from the motion of light, combined with that of the Earth in its orbit. See Light.

The objection to the Earth's motion, from there being no difcernible parallax of the fixed ftars, is anfwered by obferv- ing, that the diftance of the fixed ftars is fo great," that the diameter of the Earth's orbit bears no fenfible proportion to it i fo that the parallax is not to be difcovered by our exacteft

mflruments. Nor is this, immenfe diftance* of the fixed ftars advanced by the Copefnicans merely as an hypothefis', fb'r the fake of fulving the objection. For there feems great reafon to fuppofe the fixed ftars like to our fun, and hence to conclude their diftance to be vaftly great, fince they appear to us with fo faint a light, and of no fenfible diameter,- evert in the largeft telefcopes.

But from the diligence and accuracy of late aff.ro homers, we, learn feveral curious things in confirmation of the niotion of the Earth about the Gin'; and fcrving to (olvc this Only ma- terial objection againft it. See Light. Earths, in natural hiftory, are the various fpecies and kinds' of earthy matter found in digging, or lodged on the furface of the terraqueous globe. Thefe are either fimple, or com- pofed of entirely fimilar particles ; or compound, having a mixture of heterogeneous ones, as fand, or the like; among them.

Earths are defined to be friable, opake, infipid bodies ; not inflammable, verifiable by extream heat, diffufible in water, and feparable from it by filtration.

The fimple Earths are divided into two orders ; and under • thofe into five genera. Of the firft order are thofe Earth* which are naturally moift, of fmooth furfaces, and of a firmer texture ; and of the fecond are thofe which are naturally dry^ of rough dufty furfaces, and of a loofer texture. Of the firft of thefe orders, there arc three genera, the boles, the clays, and the marlcs ; and of the fecond there are two, the ochres, and the tripelas. The feveral diftinguiihing characters of which are thefe.

Of the firft order : 1. Boles are Earths moderately coherent; ponderous, foft, not ftiff or vifcid, but, in fome degree, ductile while moift, compufed of fine particles, fmooth to the touch, eafily breaking between the fingers, readily diffufible in water, and freely and eafily fubfid'mg from it. See Bole.

2. Clays are Earths firmly coherent, weighty, and compact, very ftiff and vifcid, ductile to a great degree while moift, fmooth to the touch, not eafily breaking between the fingers,' nor eafily diffufible in water, and, when mixed in it, not readily fubfiding from it. See Clay.

3. Maries are Earths but fiightly coherent, not ductile, ftiff, or vifcid, while moift, moft eafily diffufible in, and difunited by water, and by it reduced to a foft, loofe, inco- herent mafs. See Marie.

Of the fecond order are, 1. Ochres. Thefe are Earths fiightly coherent, compofed of fine, fmooth, foft argillacec us particles, rough to the touch, and readily diffufible in water. SeeOcHRF* 2- Tripelas are Earths firmly coherent, compofed of fine, but hard, particles, and not readily diffufible in water. The compound Earths are two; the loams, compofed of clay and fand ; and the moulds, compofed of earthy and putrified vegetable and animal matters. See Terra. Earth of Malta. See Melitensis Terra. Earth of Nocera. See Noceriana Terra. Earth of Portugal. See Portugal Earth. Lemnian Earth. See Lemni an. Livonian Earth. See Livonian Earth. Earth, among chymifts — There is plainly a fubtile Earthy which may be feparated from all animal and vegetable fub- ftances, and is the fame in all, and is the bafis of all. This remains after the feparation of the other principles by chymi- ftry, from all animal and vegetable fubftances, and neither coheres together, nor fuffers any change in the fire. Theaffayers acknowledge no difference between the Earth of animals and vegetables, but make their tefts for the niceft ufes equally of both. If water be poured on thefe afhes, they acquire fome degree of tenacity, fo as to become capable of being formed into veffels ; but if oil be added, they cohere into a much ftronger, and more compact mafs ; hence it fhould appear, that oil and Earth are the principles which give confiftence and tenacity to plants. This oil, as well as this Earth, feems the fame in all, and poffefs nothing of the poifonous or medi- cinal virtues of the peculiar plant, they being all feparable by- decoction, diftillation, and other proceffes of that kind, and not remaining in the caput mortuum, which confifts of the Earthy and this connecting oil. Long drying will divert plants of all their virtues, fo that nothing but thefe principles fhall remain in them, thefe giving folidity and figure, the plant retains its former appearance, but only thefe remaining, it poifenes none of its virtues. Bccrhaave's Chymift. part 2- p. 21. To obtain this cbymical principle pure and unmixed "from the common analyfis of vegetables, and other bodies, let the remaining mafs, after diftillation, be thoroughly calcined, then boiled in feveral waters, to get out all its fait, and after this dry'd in a clear fire, or in the fun, and the dry'd mafs will be the pure earth. Shaw's Lectures, p. 151- Earth of Dezv, an Earth much valued by many of the chy- mical experimenters, and prepared in the following manner : A large quantity of dew is to be collected, and (el in a wooden veffel, in a cool fhady place, covered with a canvas, to keep out duft and flies, there will in time come on a putre- faction in the liquor. It is fometimes three weeks, fometimes longer firft ; during the time of this putrefact ion, certain films are daily formed on the furface of the liquor, and thefe falling down to the bottom, one after the other, form, by degrees,