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has recourse to reasoning; and gives a formula for ascertaining the expansion in a certain heat.

Another irregularity in the expansion of the air is produced by the inequality of the compresing fotce; a formula for core recting this is also added. The Author mentions another corre&tion which is to be applied to the height of a mountain, as it is usually found from observations of the barometer. This arises from the diminution of gravity, in ascending or descending from the surface of the earth.

Having enumerated all the causes which produce variations in the denfity of the air, Mr. Playfair proceeds to investigate the effe&t of them all together, and after an integral calculus, maDaged with great ingenuity, he obtains a universal formula for the distance, between the two places of observation, in the following infinite feries, where, b = hyp. log, of the height of the mercury in the lower barometer, B = hyp. log. of that in the upper one, H = the height of the mercury in the lower thermometer, b = that in the upper one, m =..00245 = the expansion of the air for an increase of 1° of heat according to Fahrenheit's thermometer at 32°, r = 32, p=4342.9 &c. and g = hyp. log.

i

i trg + {r?g? + $rsg&c. ad inf. p.b-B.

itig. H+h+*g?: H + Hb + b&c. If, instead of the hyperbolical logarithms, Briggs's be used, then p becomes 10000. And the two first terms of the series will be found to be precisely M. de Luc's formula, which was discovered by that ingenious and indefatigable observer, without any inquiry into the principles on which it depends, but merely deduced from a number of observations, made in different situations, and different states of the atmosphere.

Mr. Playfair adds numerous remarks, tending to thew what particular parts of this universal expression are the corrections that must be made for particular effets. He mentions also fcveral circumstances necessary for perfecting the art of barometrical measurement, which are not get determined ; and recommends such methods as he thinks will ascertain, or at least contribute to increase our knowlege of, those facts which seem wanting to complete the theory. On the Use of negative Quantities in the Solutions of Problems by al

gebraical Equations. By William Greenfield, M. A. F. R.S. Edin. Profeffor of Rhetoric at Edinburgh.

In this paper Mr. Greenfield demonstrates the two following propofitions : ift, “Where the problem allows us to con. fider x, one of the unknown quantities, as capable of existing in two opposite situations, which may be represented by adRev. July, 1788.

D

dition

dition and fubtraction; then the equation which expresses the conditions required of x in one of these situations, and whose positive roots determine the magnitudes of * in that situation ; the same equation, by its negative roots, will determine the magnitudes of x in the opposite direction. 2d, Where the problem allows us to consider a, any of the known quantities, as capable of existing in two such opposite ficuations; then the equation which expresses the conditions of the problem, upon the supposition that a is in one of these situations, will be reduced to the equation expressing the conditions of the problem on the contrary suppofition, by simply changing the fign of the terms involving the odd powers of ai' There are not new propositions. Mr. Greenfield has however demonstrated them in a manner that we do not recollect to have seen before. Experiments and Observations upon a remarkable Cold which accom

panies the Separation of Hoar-front from a clear Air. By Patrick Wilson, M. A. F.R. S. Edin. Prof. of Astronomy at Glasgow.

In the 64th volume of our Review, p. 275, and 67th vol. p. 127, we gave an account of Mr. Wilson's experiments, by which it appeared that there is a constant difference of temperature between the snow and the air at a few feet above its surface; the snow being the coldest. In the present memoir, Mr. Wilson offers some conjectures as to the cause of this phenomenon, and adds the relation of more experiments made with a view to eftablish the facts. His general conclusions are, · That when bodies attract hoar-frost from a clear air, there is a cold produced at their surfaces; and that this cold does not originate from any peculiar qualities of bodies upon which the hoar-frost settles, any farther than as some bodies are capable of attracting from the air more or less of it in a given time. That the disposition of the air of thus parting with hoar-froft, and the cold which accompanies that separation, has a constant dependence on the general serenity of the atmosphere, and is always interrupted by the sky being overcast with clouds or fogginess, especially near the place of observation.'

Thou Mr. Wilson's experiments afforded sufficient examples of cold produced on the separation of hoar-frost from the air, he relates the particulars of one set of experiments, from which it appears that the phenomenon of an excess of cold at the surface of the snow took its rise from a manifest evaporation. To enumerate the particulars of these experiments would take more room than we can allow; we must cherefore refer the curious meteorologist to the memoir itself,

An

da Account of a Method of making a Wine, called by the Tartars

Koumiss; with Observations on its Ufe in Medicine. By John Grieve, M. D. F.R.S. Edin.

The Author of the present memoir gives the following receipt for making the Koumiss, as he obtained it from a Ruffian nobleman, who went into that part of Tartary, where it is made, for the sake of ufing it medicinally.

• Take of fresh mare's milk, of one day, any quantity; add to it a fixth part of water, and pour the mixture into a wooden vessel; use then, as a ferment, an eighth part of the foureft cow's milk that can be got; but, at any future preparation, a small portion of old Koumiss will better answer the purpose of fouring; cover the vessel with a thick cloth, and set it in a place of moderate warmth; leave it at rest 24 hours, at the end of which time, the milk will have become four, and a thick substance will be gathered on the top; then with a stick made at the lower end in the manner of a churn-staff, beat it till the thick substance above mentioned be blended intimately with the fubjacent Auid, In this fituation, leave it again at rest for 24 hours more ; after which, pour it into a higher and narrower veflel, resembling a churn, where the agitation must be repeated as before, till the liquor appear to be perfectly homogeneous; and, in this state, it is called Koumiss; of which the taste ought to be a pleasant mixture of sweet and four. Agitation must be employed every time before it be used.'

From the foregoing account, the Koumiss appears to be foured milk: on the authority of Mr. Oseretskowsky, Dr. Grieve says that it yields, by distillation, an ardent spirit.

The Author relates some cases of Phthifis and Tabes, which the use of the Koumiss had completely cured. An Improvement of the Method of correcting the observed Difance of

the Moon from the Sun, or a Fixed Star. By the Rev. Thomas Ellior.

Every attempt to facilitate or simplify the practice of aftronomy, especially when that science is applied to the art of navigation, merits the attention of mankind. Mr. Elliot's inveftigation of the problem is scientific and elegant; and his practical rule is concise. The methods now used in our navy are fufficiently exact, yet it must be confessed that their prolixity, and dependence on Tables, render them, and indeed all others, fubje&t to error or mistake. Account of a remarkable Agitation of the Waters of Loch. Tay. By

the Rev. Tho. Fleming. Loch Tay is about 15 miles long, and one broad. On Sept. 12, 1784, by an unknown cause, its waters were violently agitated, and the river which issues from it was seen to run back,

and

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and its waters for some time were dried up. Similar commow tions, though much less violent, were observed for several days. No acconis could be obtained of any earthquake in the neighbourhood, and the weather was remarkably calm, the wind being gentle, trom the N. E. and the barometer ftanding at 291. Abstract of the Register of the Weather kept at Branxholm, for Ten Years, ending Dec. 31, 1783. By his Grace the Duke of Buccleugh.

This valuable meteorological Diary contains, ift, The quantity of rain; 2d, The height of the barometer ; 3d, Of the thermometer ; 4ih, The direction of the wind. In addition to the abftract, the Duke gives a comparative view of the depth of rain at Branxholm, Ďalkeith, and Langholm, for five years. Theory of the Earth; or an Investigation of the Laws observable in

the Compostion, Diffolution, and Restoration of Land upon the Globe. "By James Hutton, M. D F R.S. Edin.

The theory of the Earth, which Dr. Hutton here delivers, is diffuse, and at the same time somewbat invelloped in obscurity. Sixteen pages are employed in pursuing,' wbat our Author calls

general or preparatory ideas,' in which he confiders the Earth as a machine, constructed on chemical as well as mechanical principles.' He says, the globe of this earth is evidently made for man;' and after some high encomiums on this lord of the creation, he adds, man, therefore, should be made the first subject of enquiry.' This enquiry, however, the Doctor confines to an ascertainment of the period in which mankind were created. The Mosaic hiftory, he says, places this beginning of man at no great distance ; and he thinks that no documents can be found in natural hiftory by which a bigh antiquity can be attributed to the human race. He then proceeds to Thew, that we possess many monuments which prove that marine animals had existed long before the human species. This inquiry gives rise to a long disquisition on marble or lime-ftone, which terminates the first part (containing preparatory ideas) of the present memoir.

The second part is . An investigation of the natural opera. tions employed in consolidating the strata of the globe. The Author fays, there are just two ways in which porous or spongy bodies can be consolidated, and by which, substances may be formed into masses of a natural shape and regular structure. One of these is fimple congelation from a Auid state, by means of cold; the other is accretion ; and this includes a separatory operation, as well as that by which the folid is to be produced. In discussing this part of his subject, Dr. H. is extremely diffuse ; he thinks that water is not the menftruum by which the consolidating matter was introduced into the interstices of Atrata, but that

con

consolidation is effected by means of beat and fusion; he supposes filiceous matter to be insoluble in water, and thence concludes that no filiceous cryftallizations, or consolidations, can be otherwise produced than by fufion; from fimiliar arguments, he concludes that all mineralization with sulphureous substances is performed by heat and fufion. The variety of sulphureo. metallic subftances, in point of compofition, is almoft indefinite; the consolidation of these heterogeneous masses cannot be performed by water, unless each of their component paris be soluble in water. The Doctor says, phlogifton, which is a principal ingredient in these compounds, refuses aqueous solution.' Does he mean, by that expression, that pblogiston cannot be united with water? Metals, especially when in their native state, he thinks, cannot ever be produced from solution, because the phlogifton is insoluble in water. A familiar experiment will thew, that phlogiston may, in solution, be detached from one body and united with another : to a solution of blue vitriol in water put a small piece of pure iron; the vitriolic acid will part with the calx of copper and seize the calx of the iron, while the phlogiston of the iron unites itself to the calx of the copper, and the precipitate will be pure copper in its metallic form.

Were we to follow the Doctor through the whole of his arguments, by which he concludes fusion to be the cause of all conSolidation, we should extend our article beyond measure, and for little other purpose than to shew the fallacy of his conclu. fions; we all therefore proceed to the third part of the memoir, wbich is entitled, Investigation of the natural operations employed in the production of land above the surface of the sea.'

Heat is again produced as the powerful agent of nature in raising the land above the waters.' Subterianeous fires are (and in this we perfectly coincide with our Author) the most probable cause of the irregularities in the surface of our present Earth, or in those internal parts which human industry bath bitherto explored*

In the fourth part of the memoir, Dr. Hutton is retrograde ; for, after having confidered those operations by which the strata of the Earth had been consolidated and then elevated above the level of the sea, he investigates the source whence those materials, from the combination of which the land was formed, were derived. In this part of his inquiry, he treads nearly the same ground with the late ingenious Mr. Whitehurft, in supposing the present Earth formed from the materials of a preceding Earth. He advances however much farther, and supposes a regular fuccestion of Earths from all eternity! and that the succession will

* See Review, vol. lxxv. p. 12, et feq. where these operations are described,

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