S26 FHILOSOPHICAL TRANSACTIONS. [aNNO 1748. 



supply the American fisheries, and all other occasions of those colonies, so as to 

 become a considerable branch of their trade. 



The author has supported all these propositions with great ingenuity ; but we 

 cannot pass over in silence the artificial means to promote the evaporation of sea 

 water, mentioned in Prop. 3, as well as to preserve the brine contained in the 

 salt pits from being diluted with rains. It will be proper, says our author, to 

 make all the salt pits of the marsh in one long row extended from east to west, 

 and for each pit to make covers of thin boards, or rather of coarse canvas, or sail- 

 cloth, stretched on frames of wood, and painted white. These covers must all 

 be fixed with hinges to strong posts and beams on the north side of the pits ; so 

 that they may be let down and drawn up with cords and pulleys, or by some other 

 contrivance, somewhat like drawbridges. These covers thus fixed may be let 

 down over the pits like a shed or penthouse in rainy weather ; and in dry wea- 

 ther may be erected almost to a perpendicular, but inclining a little towards the 

 south ; so as to form a wall with a south aspect. Thus these may serve a double 

 purpose, as coverings for the pits in wet weather, and as reflectors of the sun's 

 heat upon them in dry weather, and thus greatly promote the evaporation of the 

 aqueous parts of the brine. The hinges on which the reflectors turn may be 

 fixed about 8 or 10 inches from the ground ; by which means, when the reflec- 

 tors stand upright, there will be an opening left beneath them, through which 

 the air will continually flow in a brisk current, and greatly increase the evapora- 

 tion of the water. 



After having gone through that part of Dr. Brown rigg's work, which relates 

 to bay salt, we proceed to the methods that gentleman proposes for preparing 

 and improving white salt, which, if brought into use, may probably be of advan- 

 tage not only to private undertakers, but also to the public. For it appears, that 

 2 very different kinds of white salt are required ; the one for the use of the table, 

 and the other as a condiment for provisions. Its whiteness, dryness, and the 

 smallness of its grain, are the properties which chiefly recommend the first kind; 

 and its great strength and purity the latter.^ It is this strong and pure kind of 

 white salt, which is wanted in the British dominions ; and it is therefore our 

 author's principal design here to consider how this defect may be supplied ; though 

 at the same time instructions are given how to prepare table salt, not only better 

 in quality, but also at a less expence than it is now prepared by the common 

 methods. 



' Lemma 1. In the common processes for making white salt, the salt is deprived 

 of a considerable part of its acid spirit, by the violent boiling used in its prepa- 

 ration. — Lemma 1. Most kinds of white salt are rendered impure by the mixture 

 of various heterogeneous substances. — Lemma 3. White salt, by the violent 



