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line in diameter, carefully charged with boiling quickfilver, forms a cheap inftrument, and which may be depended upon, as to the regularity of its motions; which were found to correfpond exactly with thofe of the quickfilver in much larger tubes, even in those of near one-fourth of an inch in diameter.

Of all the methods of filling the tubes of barometers, the Author found that to be the worst (though it has been recommended by fome as one of the moft excellent) in which the mercury is introduced into the tube by means of a tunnel, with a long capillary ftem reaching nearly to the bottom of it. In a tube, thus filled, he found that the quickfilver ftood 8 lines below the ftandard. He obferves too, that the previous wafhing the internal furface of a tube with spirit of wine, is a ftill more prejudicial practice; as a barometer, thus treated, ftood an inch lower than it ought to do. But we have some reafon to fufpect that the Author has not been accurate in this experiment; particularly in the drying the tube; and that this remarkable lowness of the mercury was in fome measure owing to the elaftic vapour which is very readily generated in vacuo, in a very moderate degree of heat, from even a small quantity of moisture.

MEMOIR II. and III. Clervations on the Circulation of Air in Mines; together with an Account of the most effectual Methods of promoting it. First and fecond Memoirs. By M. Jars.

It is well known, that the inconveniences and fatal accidents attending the working of coal-pits and other mines, principally derive their origin from the flagnation of the air within them; and that the prevention of thefe dangers is only to be effected, by producing a motion in the air contained in these fubterraneous cavities: fo that the mephitic, inflammable, and other noxious exhalations, continually arifing in them, may be expelled by a current of fresh air introduced from without.

In these memoirs the Author throws confiderable light on this fubject, by offering a theory founded on a great number of obfervations, made in the different mines which he has vifited in various parts of Europe. Of thefe obfervations we fhall felect one, which is undoubtedly founded on just hydroftatical principles; and of which we may poffibly fucceed in giving an intelligible account, without the affiftance of plates. A general knowledge of the Author's theory may be of great fervice in directing thofe, who are engaged in works of this kind, where to fink the Airthafts, or other fpiracles, to the greatest advantage; and may prevent them from incurring very great and unneceffary expences, in the digging them, as it were, at random, and in fituations where they cannot poffibly, for the reafons hereafter explained, produce the beneficial effects expected from them.

In his vifits to the mines of Cheiffy, in the Lionnois, as well as to fome others which are funk under the declivity of a hill, M. Jars obferved, that, in the winter, there was a regular current of air which entered the galleries, or the horizontal paffages leading to the mine, at their mouths, which open towards the bottom of the hill; and that the air at the fame time came out of the Air-fhafts, or perpendicular paffages, that are funk down to thefe galleries, and which have their mouths towards the top of the hill. In the fummer-time, he obferved, that there was a current likewise, but that it now moved in a contrary direction; entering at the mouth of the air-fhaft, and paffing out at that of the gallery or adit . In the spring and autumn, scarce any motion of the air was perceptible; and accordingly thefe particular mines, he obferves, as well as many others, are abandoned at thefe feafons, on account of the utter impracticability of working them, arifing from this ftagnation; though the cause of it was not known.-The following manner of confidering the fubject will probably explain the cause or rationale of thefe different appearances.

We may confider the gallery at the bottom of the hill, together with the perpendicular Air-fhaft, afcending from one extremity of it, and an imaginary tube, parallel to the Airfhaft, and fuppofed to afcend from the other extremity, as a horizontal tube, to the ends of which two perpendicular tubes of equal length or height are connected, and which is proffed. by two columns of air, each extending to the top of the atmofphere. Suppofing the air contained in these two perpendicular tubes to be of equal gravity, at equal heights above the gallery; no reason can be given why the air, contained in this compound tube, fhould have a motion in any direction whatever. But if these two columns, though equal in, height, confift of a fluid, the temperature of which in one of the legs of the compound tube fometimes differs from that of the fluid contained in the other, a difference will take place in its denfity and weight; the two columns will not at thefe times be in equilibrio with each other; the heavier column will confequently defcend and the lighter will rife; in fhort, the whole fluid will be put in motion, which will continue as long as the caufes which produced it continue to exist.

Now the air contained within the perpendicular Air-fhaft (or rather that part of it which is at a certain depth below the

We speak from memory only, not having the work at hand; but there are fome obfervations, in one of Dr. Franklin's Letters. &c. on intermittent or periodical currents of air, moving alternately up and down a chimney, where there is no fire, at different times of the day, which have a near relation to the prefent fubject.

furface

furface of the ground) is at all times of the year nearly of the fame equal temperature; whereas the temperature of the column over the mouth of the gallery, which entirely confifts of the external air, varies according to the feafons. In fummer, the former, or the air contained within the perpendicular fhaft, is colder, and confequently heavier than the latter, or the external air corresponding to the mouth of the gallery. It defcends therefore along the fhaft, and produces a current which paffes out at the mouth of the gallery. In winter, on the contrary, though ftill of the fame temperature, it is relatively warmer and lighter in the Air-fhaft, and accordingly afcends; being pushed out, or upwards, by the fuperior gravity of the colder external air corresponding to the mouth of the gallery. In fpring and autumn, no motion is produced; as the external air and that of the Air-fhaft are of the fame temperature and gravity.

It follows, from this view of the fubject, that Air-fhafts, in the above-mentioned fituations, will not only be of little or no fervice in the fpring and autumn; but likewife, that if they are funk, even in great numbers, in a plain, or where the ground is nearly horizontal, no current of air will be produced through them at any feafon of the year: as the warmth, and confequently the denfity and gravity, of the refpective columns of air, in thefe different paffages, will be nearly equal to and counterbalance each other. Under fuch circumstances, however, a ventilation of mines has been propofed and practifed, which is effected by a kind of ftove placed near one of the apertures; by which the ftagnant air is extracted, at all feafons of the year indifferently, through pipes proceeding from the fire, and conveyed into the inmost receffes of the mine: in a manner fimilar to that proposed by Sutton for renewing the air in fhips, &c. The Author recommends this practice, and afterwards offers another method as a fubftitute, which, however, can only produce the fame effect at particular feafons; and that too, we apprehend, in a much smaller degree. He propofes, where the ground is level or nearly fo, a remedy to the inconvenience thence arifing; which is, to lengthen the Air-fhaft upwards, by the erection of a high tunnel or chimney over it, the fides of which fhould be very folid or thick; that the air contained within it may not be easily affected by the warmth or cold of the external air, but may, as nearly as poffible, preferve the temperature of that contained in the fubterraneous part of this prolonged tunnel: fo that the equilibrium may be destroyed between this and the other paffages, by means of the viciffitudes in the temperature of the external air; in the fame manner as where the mine is fituated in the declivity of a hill.

MEMOIR IV. On the Conftruction of Hydrometers. By M. de Montigny.

The Author's obfervations on these inftruments chiefly relate to the accurately determining, by their means, the precife ftrengths of different brandies and other fpirituous liquors; both as they are objects of trade, and as the revenue is greatly interefted in fuch determination. In his propofed improvement of the hydrometer, for this particular purpose, he principally confiders a circumftance not hitherto attended to in the construction and graduation of thefe inftruments; though the neglect of it is productive of confiderable errors. We shall not defcribe the Author's apparatus at large; but shall briefly explain the principle on which his improvements are founded.

M. Reaumur was the first who noticed the fingular phenome non, which M. de Montigny here applies to the improvement of the hydrometer. While he was employed in making the spirit thermometers known by his name, he difcovered, that when rectified fpirit, and water or phlegm, the other conftituent principle of brandy, are mixed together, there appears to be a mutual penetration of the two liquors; and not, as commonly happens in the mixture of other fluids with each other, a mere juxta pofition of parts. In fact, a part of the one fluid feems to be received into the pores of the other; fo that if a pint, for inftance, of rectified fpirit be added to a pint of water, the mixture will be very fenfibly lefs than a quart. The variations produced in the bulk of the mixed fluid, by the action of their refpective particles on each other, render the hydrometer, when graduated, as it ufually is, by equal divifions, an erroneous measure of its ftrength: as the fpecific gravity of the compound is found not to correfpond to the mean gravity of the two ingredients. M. Montigny's fcale is therefore constructed on actual obfervation of the finking or rifing of the hydrometer, in various mixtures of alcohol and water, in certain known proportions. We shall only add, that Dr. Lewis, long fince, for the very reasons that are given in this memoir, fuggefted the neceflity of thus graduating the hydrometer, for the examination of inflammable fpirits, by actual trials made in various mixtures. [See his Tranflation of Newman's Chemistry, page 450, Note r.] But whether his propofal has ever been attended to or executed in this country, we know not. MEMOIR IV. Experiments made with a View to determine the Strength of Timber. By M. du Hamel.

M. du Hamel begins this memoir by remarking, that it is become a matter of general obfervation and of universal complaint, among those interested in the fubject, that the quality or ftrength of the timber now in ufe is very confiderably infe

Engineer,

furface of the ground) is at all times of the year nearly of the fame equal temperature; whereas the temperature of the column over the mouth of the gallery, which entirely confifts of the external air, varies according to the feafons. In fummer, the former, or the air contained within the perpendicular shaft, is colder, and confequently heavier than the latter, or the external air corresponding to the mouth of the gallery. It defcends therefore along the shaft, and produces a current which paffes out at the mouth of the gallery. In winter, on the contrary, though ftill of the fame temperature, it is relatively warmer and lighter in the Air-fhaft, and accordingly afcends; being pushed out, or upwards, by the fuperior gravity of the colder external air corresponding to the mouth of the gallery. In spring and autumn, no motion is produced; as the external air and that of the Air-fhaft are of the fame temperature and gravity.

It follows, from this view of the fubject, that Air-fhafts, in the above-mentioned fituations, will not only be of little or no fervice in the fpring and autumn; but likewife, that if they are funk, even in great numbers, in a plain, or where the ground is nearly horizontal, no current of air will be produced through them at any feason of the year: as the warmth, and confequently the denfity and gravity, of the refpective columns of air, in thefe different paffages, will be nearly equal to and counterbalance each other. Under fuch circumstances, however, a ventilation of mines has been propofed and practifed, which is effected by a kind of ftove placed near one of the apertures; by which the ftagnant air is extracted, at all seasons of the year indifferently, through pipes proceeding from the fire, and conveyed into the inmost receffes of the mine: in a manner fimilar to that propofed by Sutton for renewing the air in hips, &c. The Author recommends this practice, and afterwards offers another method as a fubftitute, which, however, can only produce the fame effect at particular feasons; and that too, we apprehend, in a much smaller degree. He propofes, where the ground is level or nearly fo, a remedy to the inconvenience thence arifing; which is, to lengthen the Air-fhaft upwards, by the erection of a high tunnel or chimney over it, the fides of which fhould be very folid or thick; that the air contained within it may not be easily affected by the warmth or cold of the external air, but may, as nearly as poffible, preferve the temperature of that contained in the fubterraneous part of this prolonged tunnel: fo that the equilibrium may be deftroyed between this and the other paffages, by means of the viciffitudes in the temperature of the external air; in the fame manner as where the mine is fituated in the declivity of a hill.

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