propose also, at the fame time, to try the refiftance of fome other figures.

12. 3dly, FROM any of the numbers in the fixth column, it appears, that the altitude of a column of air, whofe preffure is equal to the refistance on the round fide of the hemifphere, is half the altitude due to the velocity of the figure; that is, half the altitude from which a body muft freely fall by gravity to acquire that velocity; and, in this inftance, agreeing with the theory. Thus, if we take the velocity of 10 feet per fecond, whose resistance in the fixth column is 12.4, we fhall have as 322 102:16:1.56 feet, which is the altitude due to to the velocity 10, and the half altitude is .78; but the weight is 12.4 ounces, which being divided by 51.14, to reduce it from the axis to the centre of the body, gives .2411 oz. for the true refistance to the convex fide. Now, a cubic foot of air weighs 1 oz.; therefore, as 1.2411: 1.1929, which is the bulk of the column of air whofe weight is equal to the refiftance, which being divided by of a foot, the area of the base, we have .86 feet for the altitude of that column, and which, therefore, is nearly equal to the half altitude above found for the velocity, exceeding it only by about the 13th or 14th part.

13. 4thly, BUT, from the fifth column, it appears, that the altitude of the column of air, whofe preffure is equal to the refistance on the flat fide of the hemifphere, is to the altitude due to the velocity of the body, as 2 to 2, instead of being equal, as required by the theory.

VII.

VII. OBSERVATIONS of the Places of the GEORGIAN

PLANET, made at Edinburgh with an Equatoreal Inftrument. By JOHN ROBISON, A. M. F. R. S. EDIN. and Profeffor of Natural Philofophy in the University of Edinburgh.

HENCE it may be deduced, (by following the method described in a paper formerly read to this Society *) that the planet was in oppofition January 13. 04. 56′ M. T. Greenwich, in longitude, 3′. 23°. 32′. 24′′ from the mean equinox, with -°. 30. 38′′ north heliocentric latitude.

THE error of the theory in longitude is nearly +5′′, and in latitude nearly 18".

I ATTRIBUTE this error in latitude to the different manner in which I obferved the declinations. I formerly obferved the difference of declination between the planet and fixed ftar by means of a common micrometer. But I was obliged to substitute Dr BRADLEY's rhombus for my micrometer, which had received an injury which I could not get repaired in time. this be allowed, the error in longitude will be diminished nearly 2′′.

If

My telescope has an achromatic object glass of 44 inches focal distance, magnifies 191⁄2 times, and takes in a distinct field of

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92.

92'. The planet was always compared with at least two stars, which passed through the field without altering the position of the inftrument. The interval between the tranfits of the fixed ftars, compared with their difference in right afcenfion in the tables, shows the error of the position of the horary wire; and the planet's difference in declination shows what portion of this error is to be applied to the time of its tranfit. When the posi tion of the horary wire was very oblique to the horizon, and the altitudes finall, a correction was made for the difference in refraction.

BOTH ends of the polar axis were firmly fupported in a ftone wall. The telescope turned round on a pin within two inches of the upper pivot of the axis, and close by the object glafs. The other end of the telescope was fupported (at the place of the wires) by a stiff rod, which turned round a pin within two inches of the lower end of the polar axis; fo that the telescope, axis and this rod, formed a triangle. Another stiff rod was fastened to the telescope at the place of the wires, with a double joint, and its other end paffed through a focket, firmly fixed on the side of the window, where it was held faft by a screw-pin. The rod was in a plane, nearly parallel to the equator. It is easy to see that, by this construction, each part of the inftrument was expofed to a longitudinal ftrain alone, and all effects of the tremor of its parts were avoided. It was fo completely free from any inconvenience of this kind, that, even in very boisterous winds, the image of the star was perfectly steady, and free from every kind of quivering. I never found any two comparisons of the planet with the same pair of ftars differ above half a fecond in time. As the inftrument was fo exact, and did not (exclufive of the telescope) cost above three pounds, I thought that this fhort account of it would be acceptable to fuch as are not provided with those expenfive inftruments which are thought effentially neceffary for making good and useful observations.

VIII. ANSWERS to the Objections of M. DE LUC with regard

to the THEORY of RAIN. By JAMES HUTTON, M. D. F. R. S. EDIN. and Member of the Royal Academy of Agriculture at Paris.

M.

[Read by the Author, Dec. 3. 1787.]

DE LUC, in his Idées fur la Météorologie, has made fome objections to the Theory of Rain* which I had the honour to lay before this Society. I fhall now endeavour to answer these objections; and hope the Society will forgive me for taking up a little of their time and attention with this fubject. The reputation of M. DE LUC is fo well established in the republic of letters, that I must not neglect remarks which have the fanction of fuch authority; although, in the present case, they appear to me to have come from a judge who was too much preoccupied with a different fystem.

THE question between us, according to M. DE LUC's own statement, is this, Whether or not, when two maffes of air of different temperatures are mixed together, the humidity of the new mafs is greater than the mean between the humidities which the two maffes had feparately? This I maintain to be a physical truth, and M. DE LUC refufes to admit it as a rule in

nature.

I HAD established this propofition, That, upon the fuppofition of the evaporating power increafing with heat, but increafing at a greater rate, the mixture of two portions of air, of different temperatures and fufficiently faturated with humidity, would: produce a condensation of water which might then become vi

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fible..

fible. I then say, That this cafe properly applies to the phenomena of breath and steam, which give a visible condenfation in mixing with the colder atmosphere; and it explains the various ⚫ appearances that may occur in mixing together feveral portions of air more or less faturated with humidity, and in different temperatures of heat and cold. For,

It is not every mixture of the atmospheric fluid, in different temperatures, that should, according to the theory, form a vifible condensation; this effect requiring, in that atmosphere, a fufficient degree of faturation with humidity. Neither is it neceffary for this effect, that the two portions to be mixed fhould each be faturated with humidity up to the temperature in which it then is found; it is fufficient, that the difference in the temperatures of those portions to be mixed fhould more. than compenfate the defect in point of faturation; but if a mixture shall be made of two portions of the atmosphere, both fully faturated with humidity, then, however small may be the difference of their temperatures, there is reafon to believe, that a condensation proportionate to this difference will take place.

HERE it is to be obferved, that I have made the rule absolute, or generalized the propofition to every supposable case; while, at the same time, I appealed to familiar examples in two cafes, that is, of humid atmosphere and of pure fteam, in giving the breath of animals in the one cafe, and the steam of a boiling kettle in the other.

THE propofition being thus made perfectly general, and concluded from experience to be a law of nature, M. DE LUC has endeavoured to refute this physical principle, by attempting to explain, in another manner, the natural appearances upon which it has been founded. It fhall now be It fhall now be my business to show, that this explanation which M. DE LUC has endeavoured to give of the subject, is founded upon nothing but inadvertency or misapprehenfion.

Hz