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The countries, however, where this is not naturally per- formed, may effect it by art, and make fa lines or fait marjhes, where they may ufe the fun's heat to great advan- tage. This is annually done to great advantage in France, and there is no reafon why we may not do the fame, either at home or in our American colonies, though it has not yet been attempted.
In order to make a feline or felt merjh, a low plat of ground rouft be chofen, adjoining to the fea, and diftant from the mouths of large rivers, and this muft be near fame convenient harbour for veflels. The ground muft be free from frefh water (brings, and out of the reach of land floods, and if poiHble fhould have a clayey bottom ; and finally, it muft be well defended from the fea, either by na- tural or artificial banks of earth of a proper ftrength and thicknefs.
The ground thus chofen muft be hollowed out to three ponds or receptacles. The firft into which the fea water is ufually admitted may be called the refervoir. The fecond receptacle, which is to be divided into three diftindt ponds, communicating with each other by narrow palTages, and containing brine of different degrees of ftrength, may be called the brine ponds ; and the third receptacle is to be fur- nifhed with an entrance, between which and the brine ponds there is to run a long narrow and winding channel ; the reft of it is to be divided into fmall and mallow pits, containing a very ftrongly faturated brine, which in them is to be converted into felt, and they may therefore very pro-v perly be called the felt pits.
The firft receptacle or refervoir muft have a communica- tion with the fea by a ditch, defended on each fide by walls of brick or ftone, and made of fuch a depth, that by it all the water of the refervoir and other parts of the felt mer/kes may be able to run out at low water, and by it alfo the fea may be admitted into it at high tide. So that at neap tides the marih may be filled with fea water to the depth of ten inches in the refervoir ; and confequently, at higher tides, to the depth of two feet, when there is occafion to overflow the marfh, as is always to be done in the winter feafon, when there is no felt to be made ; for, by this means, the wood work is kept from decav, and the clay bottom from injuries by froft. The ditch between the refervoir and the fea mull: have a flood- gate, by which the fea water may be admitted, re- tained or let out as occafion may require. Brovmrig of Salts, p. 37. Phil. Tranf. N° 51.
The fcveral ponds or receptacles muft not have all their bottoms upon the fame level, but muft be made of unequal depths, fo that the firft receptacle or refervoir may be eight inches and a half deeper than the felt pits in the third re- ceptacle. The three brine ponds alfo frtuated between the refervoir and the fait pits muft be of unequal depths. The next adjoining to the refervoir muft be the deepeft, and that which is nigheft the felt pits the fhalloweft, but all of them muft be Shallower than the refervoir ; and the three recep- tacles being thus conftructed, the water ftanding at the fame height in them all, and forming with its furface one continued plain, will be ten inches deep in the refervoir, when only an inch and half in thz felt pits. The length and breadth of the brine ponds and refervoirs are to be at difcretion ; but it is beft to err in making them too large, and in general they ought to be fo large as to furnifh the felt pits with a conftant fupply of brine, fully faturated with felt ; and, for this purpofe, it is neceffary to have diem of different dimenfions, in different countries, according to the degree of heat.
The bottoms of the refervoir and brine ponds are to be lined with any kind of lean and tough clay, or earth, that will hold water ; and the French ufe a red, or blue clay, to be had in the neighbouring grounds. With them the blue al- ways fucceeds better than the red, which colours the felt much more ; but in order to have a perfectly pure fait from this kind of manufacture, the bottom fhould be lined with fome clean cement, which will hold water perfectly, and will not be eafily broken up. Id. Ibid.
The feline or marih being thus conftructed ; the faltmen, at the proper feafon of the year, open the flood-gate when the tide is out, and drain oft all the ftagnating water ; when this is done, they repair the bottom of the marfh in feveral places, where it is found neceffary, and cleanfe the feveral receptacles from mud and dirt: after this they admit the fea water, at the next high tide, till it floats the whole marfh, and ftands at about ten inches high in the refervoir. In a day or two moft of the water in the fait pits is exhaled, and what remains is a very ftrong brine, they then let in more fea water at die two or three following tides ; and fo take care to admit as much water frefh into the marfh as has been wafted in vapour by the heat, conftantly raifing it to the height of ten inches in the refervoir, and confequently to an inch and half in the fait pits; and when the weather is extremely hot, or there are fharp drying winds, they fill them fomething higher than this.
All the parts of the marfh are thus fupplied with water out of the refervoir, but the fea water which is let into the re- Suppl. Vol. II.
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fervoif, is not confufedly let into the other water or brine of the ponds and felt pits j for, % as the feveral parts of the work communicate only by narrow channels : it is provided^ that the felt water flowing out of the refervoir never returns to it again, but gently flows along till it arrives at the fe- cond brine pond, and from that to the third, being forced forward by the fea water received from time to time into the refervoir: during this flow courfe, the watery fluid alway flies off in great quantity by exhalations, and the brine is continually preparing for cryftallization as it flows along gently, growing all the way ftronger and ftronger, as it approaches the fait pits ; fo that when it at length enters thefe pits, it is fully faturated with felt ; and particular care is taken to guard the entrance of the felt pits by a long and narrow channel, by which means the ftrong brine con- tained in thefe pits is prevented from returning back, and mixing with the weaker brine in the brine ponds ; care is alfo taken, that the ftrong brine in the felt pits is fpread out very thin, and expofed to the fun and air with a large fur- face, by which means the water more quickly exhales from it, and the felt is left concreted into cryftals. Thefe crys- tals, or felt, the workmen in France draw out every day, and difpofe them at length together in a pyramidal heap, which they cover over at the top with thatch or ftraw, and fo preferve them from the injuries of weather. Thus at a fmall expence and trouble a felt is prepared, which is found very fit for all domeftic ufes ; and France is furnifhed with a very profitable article for exportation into other coun- tries. Brownrig of Salt, p. 43. See Bay Salt, fupra.
SA-LT-petre. There are fome lands in Jamaica and our other American plantations, which are faid to contain felt-petre ; though there have been no works fet up for the refinino- and preparing it. The earth containing it has been found to have very different effe&s on vegetables from thofe of common earth.
The fugar canes that grow on' thefe fpots fhoot up much fafter, and grow larger than in other grounds ; but they rot very foon if not ground, and they do not boil fo well in- to fugar as the others. Potatoes planted in thefe grounds are always found to be two months forwarder than others, but they muft be ufed foon, otherwife they decay, the fait- pctre eating away the fkin, which is thinner on thofe roots which grow in this fort of ground than elfewhere ; and the flefhy part of this beginning to rot and moulder away. The to- bacco planted, on the fame grounds, is greatly altered in its nature alfo. It grows up very quickly, and becomes robuft: and ftrong, but it will not be cured to fo good a colour as that which grows on common earth, nor will it keep fa well; many cargoes of this tobacco have wholly perifhed at fea, by the leaves mouldering away to duft. And it is re- ported, as a certainty, that this tobacco, when fmoakedj, flames in the pipe with large flames. Phil. Tranf. N° 33.
Salt fprings. There is a copious faltfpring in the neighbour- hood of Durham, loft to the country, by its rifing in the bed of the river Weare. It does not iffue out of any one par- ticular fpot, but bubbles up from the mud and rocks for the fpace of forty yards in length, and about ten yards in breadth. It is beft examined in dry feafons, in the middle of fummer ; for at this time the water of the river is alt carried into another part of the channel, and this place left dry. The greateft quantity at this time is found to iffue out of a hard rock. It is as fait as any of the Chefhire brines ; and though but very fmall in quantity, in proportion to the fize of the river, yet it gives a manifeft faltnefs to the watef a hundred yards below. The brine has been collected in, fummer by fome curious people, and evaporated, and is found to yield a very ufeful_/a/r, not like that of the com- mon inland felt fprings, which is our white bafket fait, but rather like the bay fait, of a dufky colour, and not very pleafant tafte, but as fit for all the ufes of life as any other felt. The water of this fpring, as it runs among the ftones in fummer tinges them all of a reddifh colour, which is a property not common to other brine. Phil, Tranf. N°i6 3 .
In Chefhire, and many other counties, they make a great ufe of the water of thefe fprings, as a manure for their lands. They let out the water of thefe fprings for a certain time upon the lands, after there has been rain, and by this means the quantity of felt they contain is fo blended with the rain water, that it is too weak to hurt the corn or grafs, and yet is ftrong enough to kill worms, and other vermin. The vegetables of all kinds are improved alfo by this, for there are no lands that fatten cattle fooner, than the pa-- fture grounds which are thus, at times, overflowed by the felt water.
We have inftances of a like effect of fea fait in fmall quan- tity, and but occafionally raid on, doing great good to p«> fture in thofe high paftures about Erith, which are only overflowed fometimes in fpring tides. Thefe fatten catt)« of all kinds in a very furprifing manner. It is a wonder that no body has yet attempted a regular manuring of land with fait upon this plan, for bay felt might be had at a very fmall price in time of peace, and a bufhel or two allowed to an acre, would probably have all the advantages that thefe S s s ftVfitt
