VOL. VX.] PHILOSOPHICAL TRANSACTIONS. l6l 



spring of a watch ; and that many millions of rows of them go to make up 

 the cylinder of air, which from the top of the atmosphere presses on the mer- 

 cury in the barometer, and keeps it suspended to the height of 30 inches ; let 

 us suppose this air rarefied, so that all its springy particles expand themselves, 

 and therefore push off from this cylinder some thousands of those rows ; this 

 cylinder, now consisting of a far less number of those rows of particles, must 

 needs have a less pressure on the mercury, so that it will subside perhaps to 29 

 inches. And thus it continues till the air's spring be weakened, and so the 

 particles be crouded again into narrower room. Now if this be found to hold 

 in the theory, experience seems very well to answer it ; for I have hitherto ob- 

 served that, in cold weather and sharp frosts the mercury rises highest in the 

 barometer, and if the foreign measures agree with ours, it is usually higher here 

 than in France or Italy. 



I shall here after all subjoin two or three observations, which may serve to 

 confirm what has been said. The first is of the course of the weather under or 

 near the line. It is said in Purchas's Pilgrims, and elsewhere, that in Brasil 

 and Guiana in America ; in Guinea, Congo and Ethiopia in Africa ; in the 

 East Indies and the Maldive Islands ; they have almost continual floods of rain, 

 from about the beginning of May to the end of August, which they call their 

 winter, and the rest of the months of the year fair and clear weather, which 

 they call their summer ; so that when the sun is nearest to them, they have 

 constant rains ; and when remotest, fair weather. And this, amongst other 

 causes, may be owing to the extraordinary rarefaction of the air and lessening of 

 its specific gravity there at that time, so that the vapours in the neighbouring 

 parts of the air do all flow thither, and descend as it were in floods of rain. 

 And as this is accounted the cause of the inundation of the Nile, and some 

 other rivers, so perhaps this may be also the reason why those countries which 

 are near them, and somewhat remoter from the line, such as Egypt and the 

 like, have seldom or never any rain. The second observation is on the baro- 

 meter, viz. that when the wind is north, north-east, or north-west, the mer- 

 cury ever rises, and so the air is heavier; but when the wind comes from the 

 south, south-east, or south-west, it falls, and so the air's gravity is less ; by 

 which we may see what influence the cold and heat have on the air's weight ; 

 and a cold wind is said to fill the sails of a ship much more forcibly than a warm. 

 The 3d observation, is on an experiment of Mr. Boyle, as I find it set down in 

 the Philosophical Transactions, N° 63. I made, says he, by distillation a blood 

 red liquor, which chiefly consisted of such saline spirituous particles as may 

 be obtained from the mass of blood in human bodies : this liquor is of such a 

 nature, that if a glass phial about half filled with it be kept well stopped, the 



VOL. III. Y 



