1arded by the fides and bottom of the channel in which it moves. But it is alfo gratuitous to fuppofe, that the velocity of the cloud is the veloci. ty of the ftratum of air between the cloud and the earth; we are almost certain that it is not. It is proved by Dr HUTTON of Edinburgh, that clouds are always formed when two parcels of air of different temperatures mix together, each containing a proper quantity of vapour in the state of chemical folution. Different ftrata of air often flow in different directions for a long time. In 1781, while a great feet rendezvouzed in Leith Roads during the American war, there was a brifk E, wind for about five weeks; and, during the last fortnight of this period, there was a brifk current at the height of about of a mile.. This was diftinctly indicated by frequent fleecy clouds at a great diftance, above a lower ftratum of thefe clouds, which were driving all this time from the E. A gentleman, who was at the fiege of Quebec. in 1759 informed ús, that one day while there, blew a gale from the W. fo that the fhips at anchor in the river were obliged to ftrike their topmafts, and it was with the utmost difficulty that fome well manned boats could row against it, car rying fome, artillery ftores to a poft above the town, feveral thel's were thrown from the town to deftroy the boats; one of the hells burft in the air near the top of its flight, which was about half a mile high. The fmoke of this bomb remained in the fame pot for above a quarter of an hour, like a great round ball, and gradually diffipated by diffution, without removing many yards from its place. When, therefore, two ftrata of air come from different quarters, and one of them flows over the other, it will be only in the contiguous furfaces that a precipitation of vapour will be made. This will, form a thin fleecy cloud; and it will have a velocity and direction which neither belongs to the upper, nor to the lower ftratum of air which produced it. Should one of thefe ftrata come from the E. and the other from the W. with equal velocities, the cloud formed between will have no motion at all; fhould one come from the E. and the other from the N. the cloud will move from the NE. with a greater velocity than either of the ftrata. So uncertain then is the information given by the clouds, either of the velocity or the direction of the wind. A thick fmoke from a furnace will give us a much lefs equivocal meafure; and this, combined with the effects of the wind in impelling bodies, or deflecting a loaded plane from the perpendicular, or other effects of this kind, may give us meafures of the different currents of wind with a precifion fufficient for all practical uses.

Mr JOHN SMEATON, the celebrated engineer, has given in the Philof, Tranf. Vol. 51. the velocities of wind correfponding to the ufual denominations in our language. Thefe are founded on a great number of obfervations made by himself in the courfe of his practice in erecting wind-mills; and are as follow: Miles. per hour.

I

2

Feet. per fecond.

1.47 2193

4.40S

Names.

Light airs,

See alfo fome valuable experiments by Mr Smea top, en this fubject, in the Philof. Trans. for 1760 and. 1761.

One of the moft ingenious and convenient methods for meafuring the velocity of the wind is to employ its preffure in fupporting a column of water, in the fame way as Mr Pitot measures the velocity of a current of water. It was first propofed by Dr JAMES LYND of Windfor, a gentleman eminent for his great knowledge in all the branches of natural science, experiment, and prac tical application. His ANEMOMETER confifts of a glafs tube of the form ABCD (fig. 55.), open at both ends, and having the branch AB at right angles to the branch CD. This tube contains a few inches of water or any fluid; (the lighter the better;) it is held with the part CD upright, and AB horizontal and in the direction of the wind; that is, with the mouth A fronting the wind. The wind acts in the way of preffure on the air in AB, compreffesfit, and caufes it to prefs on the furface of the liquor; forcing it down to F, while it rifes to E in the other leg. The velocity of the wind is concluded from the difference Eƒ between the heights of the liquor in the legs. As the wind does not generally blow with uniform velocity, the liquor is apt to dance in the tube, and render the obfervation difficult and uncertain; to remedy this, it is proper to contract very much the communication at C between the two legs. If the tube has half an inch of diameter, a hole of one fiftieth of an inch is large enough; indeed the hole can hardly be too fmail, nor the tubes too large.

This inftrument gives the proportions of the velocities of different currents with the greatest precifion; for in whatever way the preffure of wind is produced by its motion, the different preffures are as the fquares of the velocities: if, therefore, we can obtain one certain measure of the velocity of the wind, and obferve the degree to which the preffure produced by it raises the liquor, we can at all other times obferve the proffures and compate the velocities from them, making proper allowances for the temperature and the height of the mercury in the barometer; because the velocity will be in the fubduplicate ratio of the denfity of the air inverfely when the pressure is the fame.

The VELOCITY of the wind is ufually eftimated that which would be acquired by falling from a

AIR is about 840 times lighter than water, and the preffure of the atmosphere fupports water at the height of 33 feet nearly. The height therefore of a homogeneous atmosphere is nearly 33 X 840, or 27720 feet. As for the velocity acquired by any fall, a heavy body by falling one foot acquires the velocity of 8 feet per fecond; and the velocities acquired by falling thro' different heights are as the fquare roots of the heights. Therefore, to find the velocity correfponding to any height, expreffed in feet per fecond, multiply the fquare root of the height by 8. We have therefore in the prefent inftance V=8√27220,=8 × 166,493

perties; in a fet of propofitions analagous to those
which form the doctrines of HYDROSTATICS. We
fhall now confider it as moveable and inert. The
phenomena confequent on these properties are ex-
hibited in the velocities which air acquires by pref-
fure, in the refiftance which bodies meet with to their
motion thro' the air, and in the impreffion which
air in motion gives to bodies expofed to its action.,
We fhall first confider the motions of which air
is fufceptible when the equilibrium of preffure
(whether arifing from its weight or its elafticity)
is removed; and next, we fhall confider its action
on folid bodies expofed to its current, and the
refiftance which it makes to their motion through
it. In this confideration we fhall adapt our invef-1332 feet per fecond.
tigation to the circumftances in which compref-
fible fluids are most commonly found. We fhail
confider air therefore as it is commonly found in
acceffible fituations, as acted on by equal and
rallel gravity; and we fhall confider it in the fame
order in which water is treated in a fyftem of

HYDRAULICS.

pa

In that science the leading problem is to deter mine with what velocity the water will move through a given orifice when impelled by fome known preffure; and it has been found, that the beft form in which this moft difficult and intricate propofition can be put, is to determine the velo. city of water flowing through this orifice when impelled by its weight alone. Having determined this, we can reduce to this cafe every queftion which can be propofed; for, in place of the pref. fure of any pifton or other mover, we can always fubfitute a perpendicular column of water or air, whofe weight thall be equal to the given preffure, The first problem, therefore, is to determine with what velocity air will rush into a void when im pelled by its weight alone. This is evidently analogous to the hydraulic problem of water flowing

cut of a veffel.

And here we must refer our readers to the for Intions which have been given of that problem,

under HYDROSTATICS, Part I. and the demonftration that it flows with the velocity which a heavy body would acquire by falling from a height equal to the depth of the hole under the furface of the water in the veffel. In whatever way we attempt to demonftrate that propofition, every ftep, nay, every word, of the demonftration applies equally to the air, or to any fluid whatever. Or, if our

readers fhould wifh to 'fee the connection or ana

logy of the cafes, we only defire them to recollect an undoubted maxim in regard to motion, that swhen the moving force and the matter to be moved vary in the fame proportion, the velocity will be the fame. If therefore there be fimilar veffels of air, water, oil, or any other fluid, all of the height of a homogeneous atmosphere, they will all run through equal and fimilar holes with the fame velocity; for in whatever proportion the quantity of matter moving thro' the hole be varied by a variation of denfity, the preffure which forces it out, by acting in circumstances perfectly fimilar, varies in the fame proportion by the fame variation of denfity. We must therefore affume it as the leading propofition, that air rushes from the atmosphere into a void with the velocity which a heavy body would acquire by falling from the top of a homogeneous atmosphere.

This therefore is the

velocity with which common air will rush into a void; and this may be taken as a standard num ber in pneumatics, as 16 and 32 are ftandard numbers in the general fcience of mechanics, expreffing the action of gravity at the furface of the earth. Greater precifion is not neceffary in this matter. The height of a homogeneous atmosphere is a variable thing, depending on the temperature of the air. If this feems any objection against the ufe of the number 1332, we may retain 8/H in place of it, where H expreffes the height of a homogeneous atmosphere of the given temperature. A variation of the barometer makes no change in the velocity, nor in the height of the homogeneous atmosphere, because it is accompanied by a proPortional variation in the denfity of the air. When it is increafed one 10th, for instance, the denfity force and the matter to be moved are changed in is alfo increafed one roth; and thus the expelling the fame proportion, and the velocity remains the fame. We do not here confider the velocity which the air acquires after its iffuing into the void by its by the 39th prop. of Newton's Principia, i. Nay, continual expanfion. This may be afcertained which appears very paradoxical, if a cylinder of air, communicating in this manner with a void, which prefies it down as the air flows out, and be compreffed by a pifton loaded with a weight, efflux with fill be the fame, however great the pref thus keeps it of the fame denfity, the velocity of fure may be for the firft and immediate effect of the load on the pilton is to reduce the air in the cylinder to fuch a denfity that its clafticity fhall exactly balance the load; and because the elafticity of air is proportional to its denfity, the denfity of the air will be increafed in the fame proportion with the load, that is, with the expelling power; for we are neglecting at prefent the weight of the included air as too inconfiderable to have any fenfible effect. Therefore, fince the matter to be moved is increafed in the fame proportion with the preffure, the velocity will be the fame as before.

It is equally eafy to determine the velocity with which the air of the atmosphere will rush into a pace containing rarer air. Whatever may be the denfity of this air, its elafticity, which follows the proportion of its denfity, will balance a proporit is the excefs of this laft only which is the mo tional part of the preffure of the atmosphere; and ving force. The matter to be moved is the fame as before. Let D be the natural density of the air, into which it is fuppofed to run, and let P be the and the denfity of the air contained in the veffel

preffure

tarded by the fides and bottom of the channel in which it moves. But it is alfo gratuitous to fuppofe, that the velocity of the cloud is the veloci. ty of the ftratum of air between the cloud and the earth; we are almost certain that it is not. It is proved by Dr HUTTON of Edinburgh, that clouds are always formed when two parcels of air of different temperatures mix together, each containing a proper quantity of vapour in the state of chemical folution. Different ftrata of air often flow in different directions for a long time. In 1781, while a great fleet rendezvouzed in Leith Roads during the American war, there was a brifk E, wind for about five weeks; and, during the laft fortnight of this period, there was a brifk current at the height of about of a mile. This was diftinctly indicated by frequent fleecy clouds at a great distance, above a lower ftratum of thefe clouds, which were driving all this time from the E. A gentleman who was at the fiege of Quebec. in 1759 informed us, that one day while there. blew a gale from the W. fo that the fhips at anchor in the river were obliged to ftrike their topmaits, and it was with the utmost difficulty that fome well manned boats could row against it, car rying fame, artillery ftores to a poft above the town, feveral fhel's were thrown from the town to deftroy the boats; one of the hells burft in the air near the top of its flight, which was about half a mile high. The fmoke of this bomb remained io the fame pot for above a quarter of an hour, like a great round bail, and gradually dilipated by diffution, without removing many yards from its place. When, therefore, two ftrata of air come from different quarters, and one of them flows over the other, it will be only in the contiguous furfaces that a precipitation of vapour will be made. This will, form a thin fleecy cloud; and it will have a velocity and direction which neither belongs to the upper nor to the lower ftratum of air which produced it. Should one of thefe ftrata come from the E. and the other from the W. with equal velocities, the cloud formed between will have no motion at all; fhould one come from the E. and the other from the N. the cloud will move from the NE. with a greater velocity than either of the ftrata. So uncertain then is the information given by the clouds, either of the velocity or the direction of the wind. A thick fmoke from a furnace will give us a much lefs cquivocal measure; and this, combined with the cffects of the wind in impelling bodies, or deflecting a loaded plane from the perpendicular, or other effects of this kind, may give us measures of the different currents of wind with a precifion fufficient for all practical uses.

Mr JOHN SMEATON, the celebrated engineer, has given in the Philof, Tranf. Vol. 51. the velocities of wind correfponding to the ufual denominations in our language. These are founded on a great number of obfervations made by himfelf in the courfe of his practice in erecting wind-mills; and are as follow: Miles. per hour.

Feet. per fecond.

Names.

trees, overturning buildings, &=.

See alfo fome valuable experiments by Mr Smea ton on this fubject, in the Philof. Tranf. for 1760 and. 1761.

One of the most ingenious and convenient methods for measuring the velocity of the wind is to employ its preffure in fupporting a column of water, in the fame way as Mr Pitot measures the velocity of a current of water. It was first propofed by Dr JAMES LYND of Windfor, a gentleman eminent for his great knowledge in all the branches of natural science, experiment, and prac tical application. His ANEMOMETER confifts of a glafs tube of the form ABCD (fig. 55.), open at both ends, and having the branch AB at right angles to the branch CD. This tube contains a few inches of water or any fluid; (the lighter the better;) it is held with the part CD upright, and AB horizontal and in the direction of the wind; that is, with the mouth A fronting the wind. The wind acts in the way of preffure on the air in AB, compreffesfit, and caufes it to prefs on the furface of the liquor; forcing it down to F, while it rifes to E in the other leg. The velocity of the wind is concluded from the difference Efbetween the heights of the liquor in the legs. As the wind does not generally blow with uniform velocity, the liquor is apt to dance in the tube, and render the obfervation difficult and uncertain; to remedy this, it is proper to contract very much the communication at C between the two legs. If the tube has half an inch of diameter, a hole of one fiftieth of an inch is large enough; indeed the hole can hardly be too fmail, nor the tubes too large.

This inftrument gives the proportions of the ve locities of different currents with the greatest precifion; for in whatever way the preffure of wind is produced by its motion, the different preffures are as the fquares of the velocities: if, therefore, we can obtain one certain measure of the velocity of the wind, and obferve the degree to which the preffure produced by it raifes the liquor, we can at all other times obferve the preffures and compate the velocities from them, making proper allowances for the temperature and the height of the mercury in the barometer; because the velocity will be in the fubduplicate ratio of the denfity of the air inverfely when the preiurc is the fame.

The VELOCITY of the wind is ufually eftimated that which would be acquired by falling from a