The author obferves, that fince Mr. Wright's map made its appearance, the globular and various other conftructions were invented, but none, according to his opinion, is fo convenient as Mr. Wright's for fea charts, because the meridians and parallels, as likewife the rhumb lines, being represented by right lines, are better adapted to the capacities of moft navigators: notwithstanding, that the parts towards the poles are extended much beyond what they are on the globe. However, he thinks that a map conftructed according to Mr. Murdoch's principles, fhews the fituation of places nearer, and is better calculated for determining fuperficial and linear measures, than the former, and that his courses also agree nearly with the computations made from the table of meridional parts, but he does not think it so easy and simple, in the practice of navigation. To this we may add, that navigators who have been used to Mercator's chart from their youth, will never approve of any other, though much better in all refpects; their attachment to the old practice being fo ftrong, that no reasoning can prevail upon them.

Article 87. An account of diftilling Water fresh from Sea-water by Wood-afbes. By Capt. William Chapman.

The captain having by accident, loft the greatest part of his flock of water, was in fear of fcarcity if the voyage fhould prove long, having neither Still nor other conveniency provided, to make fea-water fresh, by the feveral methods published by authors: neceffity, however, the mother of invention, made him try experiments, as far as his circumstances would allow; whereby he fortunately contrived a method to fupply the want: and it appears to be the most simple and practicable, that could be wifhed. It will not be difagreeable to the curious, to fee the account in his own words, as it may be of use to others under the fame circumstances.

<

6

I was not a stranger to Appleby's method; I had also a pamphlet wrote by Dr. Butler, intituled, An eafy method of procuring fresh water at fea; and I imagined, that foap might fupply the place of capital lees, mentioned by him. I now fet myself at work, to contrive a ftill; and ordered an old pitch pot, that held about ten quarts, to be made clean my carpenter, by my direction, fitted to it a cover of fir deal, about two inches thick, very clofe; fo that it was eafily made tight by luting it with pafte. We had a hole through the cover, in which was fixed a wooden pipe, nearly perpendicular. This I call the ftill-head; it was bored with an augre of one and a half-inch diameter, to

within

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 should 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 answer the end of a worm. We then cut a pewter dish, and made a pipe two ' feet long; and at three or four tryals (for we did not let a little discourage 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 cask tight, and filled it with fea-water; the pipe stuck without the cask 'three inches on each fide. Having now got my apparatus in readiness, I put seven 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 C on the head, and into it the long wooden pipe above-mentioned, which was wide enough to receive the end of the pewter one into his cavity. We eafily 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 hydrometer 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 diftillied a hogfhead of water, wholefome "and nourishing." I concluded, he had delivered this ac• count under a veil, left his method fhould be difcovered:

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

• for

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 those ashes with fea-water, he diftilled a hogfhead of fresh water, wholesome 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 afhes, with the ufual quantity of fea-water (viz. feven gallons). The refult anfwered my expectations: the water came off bright and tranfparent, with an agreeable pungent tafte, which, at first, I thought was occafioned by the afhes, but ⚫ afterwards was convinced, it received it from the refip, 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 neceffary to advise my people, not to be too free in the use of this, whilft we had any of our old flock 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 quantity of that which we diftilled. After my arrival at Shields, I invited feveral of my acquaintance on board, to tafte the 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.

C I have not the conveniency of a ftill here, or should have repeated the experiment for the conviction of some of my friends: for as to myfelf, 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 fhip bound a long voyage, was to take a small still, " with Dr. Hales's improvements, they need never want fresh 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 may ftand on the fire along with the ships boiler.'

'water.

The useful tendency of this paper, is too manifeft to need any 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 purpose above-mentioned.

5

Article

Article 97. An attempt to improve the manner of working the Ventilators, 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 uses, or to invent new ones, muft 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 ufeful 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 ufed in all nines for drawing off the water; and which I have accordingly attempted, and hope it will anfwer 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 some 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 feverally, in a more intelligent manner. illuftrates his defcription with three large copper-plates, fhewing the different parts feparately; and likewife, now 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 absolutely be the cafe in the present 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, must 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 fhould be confidered, that was the force of the steam too great for the refiftance of the boiler, it would foon deftroy the whole machine. We are fenfible, that the regulator is intended to guard against any such 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 poffible 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 most simple 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 Majesty's fhips.

If objections have been made to Mr. Sutton's scheme, it is however, no more than is done with respect to all new inventions, indiscriminately, 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 spread into a number of parts, commonly called homogeneal < rays, each of a different colour; and that these, after re'fraction, proceed from diverging: a proof, that they are dif'ferently refracted, and that light confifts of parts that differ in degrees of refrangibility.'

Might not these 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?