VOL. XV.] PHILOSOPHICAL TRANSACTIONS. 211 



fore the Spaniards, in going to the West Indies, sail southwards from Spain, 

 along the coast of Africa, till they pass beyond the tropic of Cancer, within 

 20° of the line, where they usually find an easterly wind, and then they sail on 

 westward, with full winds, so that they scarcely need to shift their sails in the 

 whole voyage ; and this they give as the reason why the voyage from Spain to 

 the West Indies is shorter, more easy, and more certain, than the return to 

 Spain. In the South-sea also, going from New Spain or Peru, to the Philip- 

 pines, or China, the voyage is easy, sailing always from east to west near the 

 line, where the easterly winds blow right a-stern. Acosta relates, that in the 

 year 1 584, there went a ship from Calloa in Lima to the Philippines, which 

 sailed 270O leagues out of sight of land, and this in two months, with a con- 

 stant breeze, their course being almost always under the line. 



Now these continual easterly winds between the tropics, I suppose, proceed 

 both from the motion of the earth and the vertical influences of the sun, in this 

 manner. As the vast fluid and aether, in which the earth floats in its annual 

 motion, moves forward with the earth in that motion, or rather carries the 

 globe of the earth along with it; so the atmosphere, and a large vortex of aether 

 beyond the moon, go round with the earth in its diurnal motion, which, though 

 according as it is removed from the earth it may be proportionably slower in its 

 motion, yet that portion of the atmosphere which is nearest the earth and sur- 

 rounds it, may be supposed to keep equal pace with the earth in its motion ; 

 and if there were no changes in the atmosphere's gravity, I suppose it would 

 always go along with the globe of the earth from west to east, in a uniform 

 motion, which would be wholly insensible to us. But that portion of the atmos- 

 phere under the line being extremely rarefied, its spring expanded, and so its 

 gravity and pressure much less than the neighbouring parts of the atmosphere, 

 and consequently incapable of the uniform motion to the east, it must needs 

 be pressed westward, and make that continual breeze from east to west between 

 the tropics. Again, on this side the tropic, about 28 or 30", there are to be 

 found constant westerly winds; and therefore the Spanish fleets from the West 

 Indies do not return the way they went, but both from Peru and New Spain 

 they sail along the coast northward till they touch at the Havannah in Cuba, 

 and rendezvousing there, they sail still higher without the tropics, where they 

 find westerly winds, which serve them till they come in view of the Azores, and 

 from thence to Seville. In like manner in the South-sea, those who return 

 from the Philippines or China to Mexico, in order to have the western winds, 

 they sail up a great height till they come right against the islands of Japan, and 

 discovering California, they return by the coast of New Spain to the port of 

 Acapulco, from whence they departed. So that though they sail easily from 



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