within three inches of the top, or extremity, where it was left folid. We made a hole in this, towards the upper part of its cavity (with a proper angle) to receive a long wooden pipe, which we fixed therein, to defcend to the tub in which the worm fhould be placed. Here again, I was at a lofs; for we had no lead pipe, nor any fheet lead on board. I thought, if I could contrive a ftrait pipe to go through a large cafk of cold water, it might anfwer the end of a worm. We then cut a pewter difh, and made a pipe two feet long; and at three or four tryals (for we did not let a little difcourage us) we made it quite tight. We bored a 'hole through a cafk, with a proper defcent, in which we fixed the pewter pipe, and made both holes in the cafk tight, and filled it with fea-water; the pipe stuck without the cafk 'three inches on each fide. Having now got my apparatus in readiness, I put feven quarts of fea-water, and an ounce ' of foap into the pot, and fet it on fire. The cover was

kept from rifing by a prop of wood to the bow. We fixed on the head, and into it the long wooden pipe above-men'tioned, which was wide enough to receive the end of the pewter one into his cavity. We cafily made the joint tight.

'I need not tell thee*, with what anxiety I waited for fuccefs: but I was foon relieved; for as foon as the pot boiled, the water began to run; and in twenty-eight minutes, I got a quart of fresh water. I tried it with an hy'drometer I had on board, and found it as light as river water; but it had a rank oily tafte, which I imagined was 'given it by the foap; this tafte diminished confiderably in two or three days, but not fo much as to make it quite palatable. Our fheep and fowls drank this water very greedily, without any ill effects. We conftantly kept our ftill at ' work, and got a gallon of water every two hours; which, if there had been a neceffity to drink it, would have been 'fufficient for our fhip's crew.

I now thought of trying to get water more palatable, and often perufed the pamphlet above-mentioned, efpecially the quotation from Sir Richard Hawkins's voyage, who "With four billets diftilled a hogfhead of water, wholesome "and nourishing." I concluded, he had delivered this ac⚫ count under a veil, left his method fhould be discovered:

This letter is addreffed to Dr. Fothergill, an eminent physician in London, and one of the people called Quikers. Capt. Chapman appears alfo to be of the fame perfuafion.

• for

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for it is plain, that by four billets he could not mean the fuel, as they would fcarce warm a hogfhead of water. 'When ruminating on this, it came into my head, that he burnt his four billets to afhes, and with the mixture of thofe afhes with fea-water, he diftilled a hoghead of fresh water, whole fome and nourishing. Pleafed with this difcovery, I cut a billet fmall and burnt it to afhes; and after cleaning my pot, I put into it a. fpoonful of those ashes, with the ufual quantity of fea-water. (viz. feven gallons). • The result answered my expectations: the water came off bright and tranfparent, with an agreeable pungent tafte, which, at first, I thought was occationed by the afhes, but afterwards was convinced, it received it from the refin, or turpentine in the pot, or pipes annexed to it. I was now relieved from my fears, of being diftreffed through want of water; yet, thought it necefiary to advife my people, not to be too free in the ufe of this, whilft we had any of our old stock remaining; and told them, I would make the experiment first myfelf; which I did, by drinking a few glaffes every day, without any ill effect whatever. This water was equally light with the other, and lathered very well will foap. We had expended our old ftock of water, < before we reached England, but had referved a good quan. tity of that which we diftilled. • I invited several of my acquaintance on board, to taste the After my arrival at Shields, water; they drank feveral glaffes, and thought it not inferior to fpring-water. I made them a bowl of punch of it, which was highly commended.

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I have not the conveniency of a ftill here, or should have repeated the experiment for the conviction of fome of my friends: for as to myself, I am firmly perfuaded, that wood-afhes mixed with fea water, will yield, when diftilled, as good fresh water as can be wifhed for: and, I think, if every ship bound a long voyage, was to take a small still, with Dr. Hales's improvements, they need never want fresh • water.. Wood-afhes may eafily be made, while there is any wood in the fhip; and the extraordinary expence of fuel will be trifling, if they contrive fo that the ftill • stand on the fire along with the ships boiler.' may

The ufeful tendency of this paper, is too manifeft to needany comment; it may, however, not be amifs to obferve, that the afhes of beech wood are faid to be preferable to any ether, for the purpofe above-mentioned.

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Article

Article 97. An attempt to improve the manner of working the Fentilators, by the help of the fire Engine. In a letter from Keane Fitz-Gerald, F.R. S.

Attempts to improve engines already made, to apply them to other ufes, or to invent new ones, muít always be acceptable to every lover of the ingenious arts. Thofe for the ufe of mines in general, either in working them to draw up the materials, or to preferve the health of the workman below, cannot be too much recommended to the public. It is well known that all mines, are fubject to noxious damps, which often fuffocate the labourers, and kill them fuddenly; or, injure their health, and destroy their lives gradually.

The author of this paper fays, That the reverend and ingenious Dr. Hales, from whom mankind has received fuch benefit by his useful application of ventilators, being inclined to extend its ufe to thofe, who work in mines at great depths under ground, where the lives of many are loft by damps and noxious vapours, occafioned by the want of a free circulation of air; and finding by ' experience, that ventilators worked by wind, do not ope' rate above one third part of the year, and in calm hot wea'ther, when moft wanted, do not operate at all; did me ⚫ the honour of applying to me for affiftance, in contriving a machine to work the ventilator, by the help of the fire ' engine, which is now generally used in all mines for drawing off the water; and which I have accordingly at'tempted, and hope it will answer the purpose.

As the lever of the fire engine works up and down al⚫ternately, and performs at a common medium, about a dozen ftrokes in a minute, it was neceffary to contrive fome way to make the beam, though moving alternately, to turn a wheel conftantly round one way, and alfo, to increase the • number of strokes to fifty or fixty in a minute.'

He

The ingenious author proceeds to enumerate the number of wheels, and other parts of which this machine is compofed, and to explain them severally, in a more intelligent manner. illuftrates his defcription with three large copper-plates, fhewing the different parts feparately; and likewife, how they appear when united together; but, as our defign is only to give a general idea of the contrivance, we must refer the curious reader who is converfant with the fubject, to the paper itself, and content ourselves with a few remarks.

This machine appears, as far as we are able to judge, rather too complicated; a fault which occafions fuperfluous

charge,

charge, and often repairs, to the increase of the expence, and hinderance of the working: this may not abfolutely be the cafe in the prefent machine, but is the general confequence of a fuperabundancy of parts. But another more real defect is, that the great force required to move it, muft unavoidably retard the motion of the fire-engine. It may be faid, that an addition of heat given to the boiler, will remove this objection; but it should be confidered, that was the force of the fteam too great for the refiftance of the boiler, it would foon destroy the whole machine. We are fenfible, that the regulator is intended to guard againft any fuch accident; yet, if the regulator ferves only for the heat required to move the fire engine, its weight must be increased in proportion to the power wanted; and it must require fome trials, before that proportion can be found. But whether it is pofiible to apply a fire-engine to ventilators without danger, is a queftion not cafily refolved in the affirmative. If a ventilator can be contrived to perform without the fire engine, and at the fame time be the moft fimple of any, it must be, of confequence, preferable to any. Such a one has been invented, by Mr. Sutton; which is now the property of Mr. Smith, and has been applied to to draw the foul air out of a great number of his Majefty's fhips.

If objections have been made to Mr. Sutton's fcheme, it is however, no more than is done with refpect to all new inventions, indifcriminately, whether good or bad.

Article 98. An account of fome Experiments, concerning the different refrangibility of Light. By Mr. John Dollond. With a letter from James Short, M. A. F. R. S.

In Mr. Short's letter to Dr. Birch, he fays, he found upon examination, that telescopes made according to Mr. Dollond's theory, are intirely free from colours, and are as diftinct as reflecting ones. It is well known,' fays the author, that a ray of light, refracted by paffing through mediums of different denfities, is at the fame time proportionally divided, or fpread into a number of parts, commonly called homogeneal C rays, each of a different colour; and that thefe, after refraction, proceed from diverging: a proof, that they are differently refracted, and that light confifts of parts that differ in degrees of refrangibility.'

Might not thefe different degrees of refrangibility, proceed from different rays falling upon the fame point of the refracting furface, coming from different points of the fun's furface, and therefore, falling upon that furface in different

2

angles?

angles? For it has not, that we know of, been proved, that all the rays of light iffuing from the fun, proceed in the directions of radii, drawn from the center through the points in the surface, or rather, in a direction perpendicular to the tangent of the furface of the fun. But even fuppofe that they did, might not the particles of our atmosphere refract some, fo as to meet others in the fame point, in the refracting furface? This however, is only a conjecture, and does not affect the author's theory, which fhews what effect these rays produce, when refracted by different mediums.

He proceeds; Every ray of light paffing from a rarer into a denfer medium, is refracted towards the perpendicular; but from a denfer into a rarer one, from the perpendi'cular; and the fines of the angles of incidence and refraction are in a given ratio. But light confifting of parts, which are differently refrangible, each part of an original, or compound ray, has a ratio peculiar to itfelf; and there'fore, the more a heterogene ray is refracted, the more will 'the colours diverge, fince the ratios of the fines of the homogene rays are conftant; and equal refractions produce • equal divergencies.'

The first part of this paragraph, is demonftrated from the general law of attraction; fince all particles of matter attract each other in proportion to their quantity directly, and as the fquares of their distances inversely; fo that a ray of light falling obliquely upon a furface, will be bent more or lefs, as the denfity of the substance of which this furface, or rather folid, is greater or lefs : and as to the latter part, it has been demonftrated by all optical writers.

The Author obferves, that it has been hitherto fuppofed, that the divergency of the colours is the fame under equal refractions; which he thinks is not always true. This he endeavours to prove by fome experiments, which we shall mention hereafter, and then he proceeds, as no mediun is known, which will refract light without diverging the colours, and as difference of refrangibility feems thence to be a property inherent in light itfelf, opticians have, upon that confideration, concluded, that equal refractions muft produce equal divergencies in every fort of medium whence it fhould alfo follow, that equal and contrary refractions must not only deftroy each other, but that the divergency of the colours from * one refraction would likewife be corrected by the other; and there could be no poffibility of producing any fuch thing as refraction which would not be affected by the different refrangibility of light; or, in other words, that however a REVIEW, Aug. 1759.

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