cated air of our atmosphere feemed almoft (probably wholly) of this kind; and the diminution did not, in any degree, arife from burnt matter, but entirely from the change into acid. Yet the folution of litmus was fo far decompofed as to lofe its colour, and to form at laft fixed air. As the author has not indulged himself in drawing any confequences from these difcoveries, we shall not intrude on his province, or our readers patience yet, if the theory be established, numerous phænomena may probably be explained from it, particularly the revivifying effects of thunder-ftorms, the production of nitre, and many others.

Art. XXIV. An Account of a Measurement of a Bafe on Hounslow-Heath. By Major-general William Roy, F. R. S. and A. S.-This is a national work of very confiderable importance; and it feems to have been executed with the judgment and accuracy which the great object required.

In the beginning of October, 1783, count d'Adhemar, the French ambaffador, tranfmitted to Mr. Fox, then one of his majefty's principal fecretaries of ftate, a memoir of M. Caffini de Thury, in which he fets forth the great advantage that would accrue to aftronomy, by carrying a feries of triangles from the neighbourhood of London to Dover, there to be connected with thofe already executed in France, by which combined operations the relative fituations of the two most famous obfervatories in Europe, Greenwich and Paris, would be more accurately afcertained than they are at prefent.

This memoir the fecretary of ftate, by his majesty's command, tranfmitted to fir Jofeph Banks, the very refpectable and worthy prefident of the Royal Society; who, about the middle of November, was pleafed to communicate it to me, propofing, at the fame time, that I fhould, on the part of the Society, charge myfelf with the execution of the operation, To this propofition I readily affented, on being foon afterwards affured, through the proper official channels, that my undertaking it met with his majefty's moft gracious approbation,'

We must fuppofe our readers acquainted with the operation; for, if they were not, it would exceed our limits to explain it. Those who are acquainted with trigonometry know, that if the bafe of a triangle be accurately measured, angles are afcertained with the greatest nicety, particularly in the improved state of our mathematical inftruments; and, having obtained the dimenfions of one triangle, a feries of triangles, formed from different visible objects in fucceffion, can be carried on to any diflance, and meafured with the greatest exactness. But if we err in measuring the bafe, an inconfiderable error is multiplied, and becomes of confequence, when the triangles are numerous, and the distance great; fo that the most minute and anxious attention is required in this operation. We have

great

great reason to think that a work of this kind has never, before this time, been fo perfectly executed. The gentlemen, entrusted with the operation, have avoided almoft every poffible cause of error; and, by their precautions, have greatly elucidated feveral natural effects. It is impoffible that we can give even the general detail of their methods, but fhall enlarge, fomewhat, on fome circumftances which occurred to them in their progrefs.

The chain made by Mr. Ramfden anfwered exceedingly well; and we would recommend this form even in common. menfuration. It has always been thought that fir, cut in the direction of the fibres, was not liable to be affected by moisture; and the choiceft rods of this kind, both of New England fir, and Riga wood, thoroughly feafoned (for they were cut from old mafts) were employed, but they were found to be affected confiderably (we now speak of a very delicate experiment) by moisture; and the ufual corrections were uncertain, for fome. times they exceeded, and at others fell fhort. Metallic rods were too heavy; but they at laft procured glass tubes twenty feet long, and perfectly ftrait. With thefe tubes the bafe was ultimately measured. We fhall infert a fhort extract, relating to the remarkable agreement between the glafs rods and the chain.

In this manner we proceeded, and in the course of the day were only able to measure the length of ten chains, or one thousand feet, being the forty-fixth and forty-fifth hypothenufes of the bafe, the first of four hundred and the laft of fix hundred feet. Being arrived at this point it was found, that the fine line on the brafs flide, marking the extremity of the tenth chain, fell short of another fine line on the fame flide, denoting the end of the fiftieth glass rod, juft two-tenths of an inch. Now it will appear hereafter, when we come to fhew, by the experiments with the pyrometer, what the real contractions of the chain and glass rods were, for the degrees of difference of temperature below that in which their refpective lengths were laid off, that this fmall apparent difference of two-tenths of an inch, between the two modes of meafuring the thousand feet, fhould have been 0.17938 in. to have made the two refults exactly agree, which is a real difference of only 0.02c62of an inch. Suppofing then every thousand feet of the base to have been measured by the chain with the fame attention, and confequently with the fame, or nearly the fame fuccefs (and there furely cannot be any reafon to doubt of the practicability) we fhall have 27.404 × 0.02062 in. = 0.565 in. or a defect of fomething more than half an inch on the whole length of the bafe.'

The defcription of what is called the microfcopic pyrometer, employed to determine, by experiment, the expanfion of

Y 4

the

the metals concerned in measuring the bafe, then follows: it is alluded to in the above extract. This inftrument also we are unable to defcribe; but, after the feveral corrections, the measure of the base reduced to the level of the fea, and making a portion of the mean circumference of the earth, amounts to 27404.7219 feet.

By

Art. XXV. Abstract of a Register of the Barometer, Thermometer, and Rain at Lynddon, in Rutland, 1784. Thomas Barker, Efq. Alfo of the Rain at South Lambeth, Surrey; and at Selbourn and Fyfield, Hampshire. By Thomas White, Efq. F. R. S.-The year 1784 was remarkable for the feverity of the weather, with which it commenced. In February, the lowest point of the out-door thermometer was 9°; but there is little information from this mode of keeping a journal. Two obfervations fhould be made every day, and each day's weather particularly fpecified. October, as ufual, was dry; but the rain that month, in 1785, compensated for the October droughts of the preceding years.

The volume is concluded, in the common way, with a lift of prefents; and we must conclude this account by acknowledging our obligations for much inftruction contained in the fecond part, which has been now the subject of our remarks.

The Chemical Efays of Charles William Scheele. Translated from the Tranfactions of the Academy of Sciences at Stockholm. With Additions. 8vo. 6s. in Boards, Murray.

THIS tranflation owes its origin to a conversation on the flow gradual manner in which the advances in literature on the continent are communicated to this ifland. We have often lamented it, in various parts of our Journal, for it has expofed us to the very difagreeable circumstances of correcting authors for errors, which they would have avoided by extending their views beyond the British publications, and of detracting from affumed merit, when they have only hinted at what, in other works, has been fully explained. On this account, we have allotted a large part of our Journal to that science, which is daily augmenting in the number of its facts, and becoming of more confequence by the importance of its discoveries. We have lately too, pointed out fome useful pub. lications, which fhould be made our own, by an English drefs, and this kind of recommendation will be occasionally repeated; fo that we hope this nation, which has hitherto led the way in science, will not be afhamed, in fome inftances, to follow. There is only one objection to his plan and ours; the reader may exclaim, inopem me copia fecit.' A stock

of

of knowledge at once put into his hands; another world opened to his view, may terrify the indolent, and make even a moderately fanguine enquirer hefitate to attempt a ftudy almoft new. The objection will, however, be temporary; and should, at least, not impede defigns, which must be attended with so many advantages.

Mr. Scheele is chiefly known in England, by his treatise. on air and fire,' tranflated by Mr. Forster, and adorned with the remarks of Mr. Kirwan and Dr. Priestley. His other works are mentioned by Bergmann and Fourcroy, by Dr. Priestley, Mr. Cavendish, and fome of the more learned English chemifts; but his opinions have been gradually introduced, fo that many of them are commonly known, even by men who have fcarcely ever heard of the author. The tranflator is known by the Treatife on Elective Attractions, lately published in English, and elucidated by valuable notes; we suspect also, that we are indebted to him for the Differtations of Spalanzani; and we have had occafion to praise his execution of both thefe tasks. In the work before us, he rather appears to have been a corre&or and editor than a tranflator. The tranf lation was executed by a foreigner, and a few foreign idioms ftill occur; but not fo many as to render the Effays unpleafing to an Englishman. Mr. Beddoes has, however, added other papers from meffieurs Crell, Meyer, Wiegleb, &c. which render the Effays more valuable. A French tranflation, by M. Morveau, is mentioned, which the editor has not feen. We have not been able to compare it with the prefent verfion. It is not tranflated by M. Morveau, but by madame P * * *, of Dijon, and is published in three volumes, duodecimo. The notes are curious and ufeful; which Mr. Beddoes purposes to add to his tranflation of the fecond volume of Mr. Scheele's other Effays.

The four first Effays are on the Fluor Mineral and its Acid. The various fteps which led to the prefent opinions, and the errors which once mifled chemifts, are carefully explained. The fluor acid was fuppofed, in union with water, to form flint; and this opinion is not yet wholly forfaken, though it is certainly not tenable. We have, on different occafions, fuggefted fome doubts relating to it, and pointed out its probable origin, viz. the glass-veffels employed in the operation. In fact it is clearly fhewn, in the third and fourth Effay, that the acid diffolves the glafs, and carries the flint over into the receiver, where it is depofited on coming in contact with water. Yet fo intimate is their union, that, in the form of air, the acid and fint pafs through quickfilver, unchanged. But the appear

ance,

ance, during the change, when the air comes into contact with water, is fo obviously that of a depofition, and fo little refembling a new formation, that we always hefitated about the common opinion, even before we faw the Effay in which the caufe is explained.

It is well known that we are indebted to Mr. Scheele for our knowledge of the properties of manganefe. His Essay on this fubftance is very full and fatisfactory. We fhall tranfcribe an abstract of its different properties, in our author's own words.

Manganefe has a strong elective attraction for all phlogistic fubftances. This attraction becomes stronger,, if there be a menftruum prefent which at the fame time can unite with the phlogisticated manganefe. In this fituation the manganese is able to attract phlogifton more strongly than the nitrous acid does via humida. When manganefe is faturated with phlogifton, it lofes its black and affumes a white colour, which, however, difappears as foon as the phlogifton is feparated from it again. Without combining it with phlogifton, there is no way of producing a colourless folution of manganese in any acid; and, whenever phlogiston is wanting, the folution becomes blue or red. By means of thefe four general qualities of manganefe, discovered by a train of experiments, all its known effects are easily explicable.

Its ftrong attraction for phlogifton makes it the most proper fubftance to dephlogisticate the marine acid, which then chiefly appears in the form of air; for, when feparated from phlogifton, it unites with water in a very small quantity, giving it only a tafte flightly acid. Yet it should be noticed, that the lofs of phlogiston is not effential to its aerial form; fince muriatic acid air, made in the common way, will combine in any degree with water, and form common muriatic acid, though this union is formed flowly, and fometimes imperfectly. We shall add the properties of this dephlogisticated acid, as they are not generally known."

I obferved that the corks (A) became yellow within the receiver, as from aqua fortis. and the lute was likewife corroded during the diftillation. (B) Paper coloured with lacmus became nearly white; all vegetable red, blue, and yellow flowers, grew likewife white in a fhort time; the fame thing happened to green vegetables: meanwhile, the water in the veffel was changed into a weak but pure muriatic acid. (C) The former colour of the flowers, or of the green vegetables, could not be recovered either by alkalis or acids (D), expreffed oils and animal fat, whether dropped into the tube, or fmeared upon it, grew in a fhort time as tenacious as turpentine. (E) Cinnabar grew white upon the furface, and, when it was washed in water, € a pure