With regard to the saltness of the sea water, it is very rationally judged to arise from great multitudes both of mines and mountains of salt, dispersed here and there in the depths of the sea. The salt being continually diluted and dissolved by the water, the sea becomes impregnated with its particles through out ; and for this reason the saltness of the sea can never be diminished Dr. Halley supposes, that it is probable the greatest part of the sea salt, and of all salt lakes, as the Caspian Sea, the Dead Sea, the Lake of Mexico, and the Titi caca, in Peru, is derived from the water of the rivers which they receive ; and since this sort of lakes has no exit or dis charge but by the exhalation of vapours; and also since these vapours are entirely fresh, or devoid of such particles ; it is certain, that the saltness of the sea and such lakes must, from time to time, in crease, and therefore, the saltness at this time is greater than at any time hereto fore He further adds, that if, by experi ments made in different ages, we could find the different quantity of salt which the same quantity of water (taken lip in the same place, and in all other the same circumstances) would aflOrd, it would be easy from thence, by rules of proportion, to find the age of the world very nearly, or the time wherein it has been acquiring ita present saltness.
The Bishop of Landaff has recommend ed a most simple and easy mode of as certaining the saltness of the sea in any latitude : we insert it in his own words.
"As it is not every person who can make himself expert in the use of the common means of estimating the quan tity of salt contained in sea-water, l will mention a method of doing it, which is so easy and simple that every common sailor may understand and practise it, and which, from the trials I have made of it, seems to be as exact a method as any that has yet been thought of. Take a clean towel, or any other piece of cloth, dry it well in the sun, or before the fire, then weigh it accurately, and note down its weight ; dip it in the sea-water, and when taken out, wring it a little, till it will not drip when hung up to dry ; weigh it in this wet state, then dry it either in the sun, or at the fire, and, when it is perfectly dry, weigh it again. The excess of the weight of the wet. ed cloth above its original weight, is the weight of the sea-water imbibed by the cloth ; and the excess of the weight of the cloth after being dried, above its original weight, is the weight of the salt retained by the cloth; and by comparing this weight with the weight of the sea water imbibed by the cloth, we obtain the proportion of salt contained in that spe cies of sea-water.
" Whoever undertakes to ascertain the quantity of salt contained in sea-water, either by this or any o: her method, would do well to observe the state of the wea ther preceding the time when the sea water is taken out of the sea, for the quantity of salt contained in the water near the surface may be influenced both by the antecedent moisture and the ante cedent heat of the atmosphere."
Whether the sea is salter or not at dif ferent depths, has not yet been properly ascertained ; but that its temperature varies considerably in proportion to the depth, we have decisive proof.
" With respect to the temperature," says Bishop Watson, "of the sea at dif ferent depths, it seems reasonable enough to suppose, that in summer time it will be hotter at the surface than at any con siderable depth below it, and that In win ter it will be colder.
" Mr Wales describes the instrument he made use of for trying the tempera ture of the sea at differeat depths in the following terms : 'The apparatus for try. ing the sea-water at different depths con sisted of a square wooden tube, of about eighteen inches long, and three inches square externally. It was fitted with a valve at the bottom, and another at the top, and had a contrivauce for suspend ing the thermometer exactly in the mid dle of it. When it was used, it was fas tened to the deep sea-line, just above the lead, so as all the way as it descended, the water had a free passage through it, by means of the valves which were then both open ; but the instant it began to be drawn up, both the valves closed by the pressure of the water, and of course the thermometer was brought up in a body of water of the same temperature with that it was let down to' With this in strument, which is much the same with one formerly described by Mr. Boyle, in his observations about the saltness of the sea, water was fetched up from different depths, and its temperature accurately noticed, in different seasons and lati tudes.
" August 27, 1772, south latitude 24° 40'. The heat of the air was 721, of the water at the surface 70, of water from the depth of 80 fathoms 68.
" December 27, 1772, south latitude 58° 21'. The heat of' the air was 31, of the water at the surface 32, of water from the depth of 160 fathoms 33i.
" In the voyage to the high northern latitudes before mentioned, they made use of a bottle to bring tip water from the bottom, which is thus described; ' The bottle had a coating of wool, three inches thick, which was wrapped up in an oiled skin, and let into a leather purse, and the whole inclosed in a well pitched canvass bag, firmly tied to the mouth of the bottle, so that not a drop of water could penetrate to its surface. A bit of lead, shaped like a cone, with its base downwards, and a cord fixed to its small end, was put into the bottle ; and a piece of valve leather, with half a dozen slips of thin bladder, were strung on the cord, which, when pulled, effectually corked the bottle on the inside' We have here put down two of the experiments which were made during that voyage.