« AnteriorContinuar »
ways equable, and, at all times, equal to the mean motion; whence the place of the fun being known at any certain time fixed, his place, at any other time affigned, would be readily computed. But Aftronomers, by comparing together the places of the fun deduced from obfervation, have found, that the apparent motion of the fun through the ecliptic is unequal, and that he moves swifter through fome parts of it than through others that his apparent motion is, fometimes, 61 minutes nearly, at others, fcarce 57 minutes; and that he is nearly seven days longer in moving from the first point of Aries through the northern half of the ecliptic, to the first point of Libra, than from thence through the fouthern half of the ecliptic, to the first point of Aries.
The ancient Aftronomers, who allowed of no other motion in the heavens than what were circular and equal, that they might account for these inequalities in the fun's motion, and adjust the feveral quantities of it in feveral parts of the orb, fuppofed that the fun moved round the earth in a circular orb, but excentrical; that is, whofe center was at fome distance from the center of the ecliptic, in which they placed the earth; and that this circular orb was described by an equal motion, so that a line or ray drawn from the center of the orb to the fun, defcribed equal areas in equal times, 2 A
But the great Kepler, by comparing the obfervations of the famous Tycho together, difcovered, that Mars was not carried round the fun in a circular, but in an elliptic orb; that the fun was placed in one of the foci of that ellipfis, and that in mov ing round the fun his motion was fo regulated, that a ray of light drawn from the fun to the planet, described an elliptic area, or space, always proportionable to the time. This induced him to examine whether the motions of the other planets were regulated by the fame law; and having fatisfied himself that they were, he concluded, that it was reasonable to fuppofe, that the earth also observed the fame law, and moved round the fun.in an elliptic orb; and this having been confirmed by all obfervations made fince his time, there is no room left to doubt of the truth of it.-And, therefore, as the earth in her annual motion round the fun, is governed by the equal and uniform defcription of areas, which increase and decrease uniformly with the time, it is impoffible the can every where move with the fame uniform velocity, but it must be constantly changed; and that in every different part of her orb the will acquire different degrees of velocity; wherefore, to determine her true place at any given time, we must find the pofition of a right line, which paffing through one of the foci
of the ellipfis, will cut off a trilineal area defcribed by its motion, to which the whole area of the ellipfis fhall have the fame proportion that the periodical time of the earth has to any other given time: which pofition being found, we shall have the place of the earth, at the given point of time.
This problem was firft propofed by Kepler, after he had difcovered the laws of uniform areas, and hence called by. Aftronomers Kepler's Problem, the folution of which Mr. Stewart has given in the paper before us. Several Mathematicians have, however, folved it before Mr. Stewart, but not without having recourse to the higher Geometry, or very operofe methods of calculation; both which this Gentleman has avoided and fol ved the problem in an easy, perfpicuous, and strict well geometrical method,
Art. 7. Of the Gold produced by evaporating Fluids, and of fome
From feveral experiments made by Dr, Cullen, it appears, that the power of evaporating fluids in producing cold, is nearly according to the degree of volatility in each; and that the cold produced, is the effect of evaporation,
Art. 8. Contains experiments upon Magnefia, Quick-lime,. and some other alcaline fubftances. By Jofeph Black, M. D. This Gentleman's motive for undertaking thefe experiments, and candid acknowlegement of his difappointment, appear truly laudable. My curiofity," fays he, led me, fome time ago, to enquire more particularly into the nature of Magnefia, and especially to compare its properties with thofe of the other abforbent earths, of which there plainly appeared to me to be very different kinds, altho' commonly confounded together under one name. I was, indeed, led to this examination, partly by the hope of difcovering a new fort of lime, and Time-water, which might poffibly be a more powerful folvent of the ftone, than that commonly ufed; but was disappointed in my expectations.' The f following process is given to prepare Magnefia. Diffolve equal quantities of Epfom falt and of pearl afhes, fepa rately, in a fufficient quantity of water; purify each folution from its dregs, and mix them accurately together by violent agitation: then make them juft to boil over a brifk
• Add now to the mixture three or four times its quantity of hot water; allow the Magnefia to fettle to the bottom, and ⚫ decant off as much of the water as poffible. Pour on the
• fame quantity of cold water; and after fettling, decant it off in the fame manner. Repeat this washing with the cold • water ten or twelve times; or even oftner, if the magnesia be required perfectly pure for' chemical experiments.
When it is fufficiently wafhed, the water may be ftrained and fqueezed from it in a linnen cloth; for very little of the magnefia paffes through.'
As a proof of the medicinal efficacy of magnefia, the Doctor informs us, that he made a neutral falt of magnesia and diftilled vinegar; chufing this acid, as being, like that in • weak ftomachs, the product of fermentation.' Six drachms of this being diffolved in water, was given to a middle-aged man, with directions to take it by degrees. After having ⚫ taken about a third, he defifted, and purged four times in an eafy and gentle manner. A woman of a strong conftitution got the remainder as a brifk purgative, and it operated ten times, without caufing any uneafinefs. The taste of this falt is not difagreeable, and it appears to be rather of the cooling, than of the acrid kind.'
From hence Dr. Black proceeds to an invefigation of the chemical properties of magnesia. His experiments to this purpofe are well planned, and feem to have been accurately conducted; but they are fo numerous, as to extend to upwards of fixty pages, and at the fame time fo mutually dependent on each other, that we must refer the inquifitive reader to the original.
Art. 9. Of the Analysis and Ufes of Peats by Alexander Lind, Efq;
This is far from being one of the leaft important articles in this collection: the particular defign of it is, to render the fubject treated of more extenfively ufeful. Befides the purposes for which it has been commonly employed, Mr. Lind conceives it might be advantageoufly used for smelting iron and other The method he recommends is, to bring them to be as folid and compact a fubftance as poffible. The densest bodies,' he obferves, cæteris paribus, when thoroughly heated, are the hotteft: hence it is, that metals, as they 6 are the heavieft bodies, fo they reach the greateft degree of heat. The fame holds in fewel; the hardest woods are made choice of when a ftrong heat is wanted; and even in common peats I have fhewn you how far preferable the hard and folid are to the light and fpungy. By fome experiments which I have made, I find it to be no difficult matter to bring peat to a confiderable degree of folidity, as you yourselves
may fee, by the fpecimen I now fhew you. The fimple • operation of grinding does the business; and as a peat, when <taken out of the mofs, is a foft body, and easily grinded, a machine may be eafily contrived to grind, at a moderate expence, several tons in a day. The charge of digging peats, cutting them into fquares, or the form of bricks, when ⚫ of a proper drynefs, will be little different from that of making peats in the ordinary way. The folidity of peat prepar ed in the manner mentioned, is furprizing; its specific gra'vity being fomewhat greater than that of pit-coal.’
The advantages of peat-afhes and peat-duft, for manure, are pretty well known in South, as well as North Britain. But as this gentleman has propofed another method of using peat, for the melioration of land, what he has advanced on this fub, ject may prove a serviceable hint to fuch as have fandy farms, and are fituated in the neighbourhood of this commodity. Peatmofs,' he fays, 'being wholly a vegetable matter, muft, if reduced to a thorough ftate of putrefaction, anfwer the fame purposes of • fertilizing ground, as other putrified vegetables. While it lies in the mofs, there is too great a quantity of water, to <raife a fufficient degree of heat, to bring the vegetables of ⚫ which peat-mofs is compofed, whether actually growing, ⚫ decaying, or decayed, to a complete degree of putrefaction.
But if it were taken out of the mofs, and laid in heaps, like < other vegetables, to rot, with a degree of moisture fuit⚫able for that purpose; and if, to begin, and alfo quicken, the putrefaction, green, fresh, fucculent plants were employed in a fufficient quantity firft to raise a heat; this I make no doubt would, by communicating it to the mossy subftance, in a fuitable time, and by right management, reduce the whole mafs to the ftate defired.'
The mention of two other ufes of peat concludes this article; the one is, that peat-duft ftrewed upon ground where peas, or other feeds are fown, in order to have an early crop, is ⚫ an excellent prefervative of fuch vegetables from the froft; as it keeps the ground warm.' The other is, that there is nothing properer than peat to ftop water, and to confine it, in the making of fish-ponds,' &c.
Art. 10. The Effects of Semen Hyefciami Albi, by Dr. Archibald Hamilton, Phyfician in Edinburgh.
The effects here defcribed, are fuch as might naturally be expected from an over-dofe of any narcotic vegetable: but what makes this cafe the more remarkable, is, that the patient
had accustomed himself to the ufe of white hen-bane feed, in order to procure fleep, for two years before, and that the quantity at this time taken, did not exceed twenty-five grains.ɔr'] baliovintos Art. 11. The effects of the Thorn-apple, by Dr. Abraham Swaine, Phyfician at Brentford,quor
In this cafe the tramonium was gathered, and taken, inftead of the fruit of the Burdock, which is faid to have been advised as a remedy for the gravel. The confequence that happened is not near fo unaccountable, as that fuch a mistake could be made. It alfo is not impoffible, that the patient might further misunderstand his adviser, and look for fruit, inftead of a root; the latter being a common prescription for complaints of that fort.
Art. 12. The effect of Musk in curing the Gout in the Stomach; by James Pringle, Efq; late Surgeon to the third régiment of foot-guards.
The cafe is thus related. A gentlewoman aged fortythree years, naturally of a delicate conftitution, who has • been for feveral years subject to hylleric fits, attended with a dry asthma, which her fhape much contributed to; was fre· quently attacked, to a violent degree, with the gout in her head and ftomach, as well as in all her extremities; and with which she was lame the moft part of Summer, 1745. • On the 3d of November following, fhe was violently feized < with it in her ftomach, which occafioned violent hiccups and convulfions of the part. The defcription fhe gave of it was, that as foon as these fits feized her, there came on a violent working of her ftomach, and fo great an agitation of her back, that her maid was not able to keep her hand on it. By degrees it rofe to her throat; when the was almost strangled. She could by no means lie down, but was forced to fit night and day in an eafy chair; and even then, if the leaned her head to the one fide or the other, it gave her great pain, fo that he was obliged to fit in an erect pofture, Her legs were very much (welled, which fubfided a little on laying them on a chair; but as foon as that happened, the afthma returned.? In this condition Mr. Pringle found his patient on the 21ft of November; when he ordered her the following bolus ;
R Cinnab. nativ.
Mofch. opt. gr. xvi.
Syr, balf. q. f. F. Bolus.