THE GULF STREAM. 35 



104. It is immaterial, so far as the correctness of tlie principle 

 Evaporation and pre- i^pon which tliis rcasomng depcncls is concerned, 

 cipitation. wlietlier the annual evaporation from the trade-mnd 

 regions of the Atlantic be fifteen, ten, or five feet. The layer 

 of water, -whatever be its thickness, that is evaporated from this 

 part of the ocean, is not all pom^ed back by the clouds upon the 

 same spot whence it came. But they take and poui* it down in 

 showers upon the extra-tropical regions of the earth — on the land 

 as well as in the sea — and on the land more water is let down than 

 is taken up into the clouds again. The rest sinks do^n tln^ough 

 the soil to feed the springs, and retmTis tlirough the rivers to the 

 sea. Suppose the excess of precipitation in these extra-tropical 

 regions of the sea to amount to but twelve inches, or even to but two 

 — it is twelve inches or tvro inches, as the case may be, of fresli water 

 added to the sea in those parts, and which therefore tends to lessen 

 the specific gravity of sea-water there to that extent, and to produce 

 a double dynamical effect, for the simple reason that what is taken 

 fi'om one scale, by being put into the other, doubles the difference. 



105. Now that we may form some idea as to the influence which 

 cun-ent into the ca- the salts left by the vapour that the trade-winds, 

 ribbean Sea. north-cast and south-east, take up from sea-water, is 

 calculated to exert in creating currents, let us make a partial cal- 

 culation to show how much salt this vapour held in solution before 

 it was taken up, and, of course, wdiile it was yet in the state of sea- 

 water. The north-east trade-wind regions of the Atlantic embrace 

 an area of at least three million square miles, and the yearly eva- 

 poration from it is (§ 103), we will suppose, fifteen feet. The salt 

 that is contained in a mass of sea-water covering to the depth of 

 fifteen feet an area of three million square miles in superficial extent, 

 would be sufficient to cover the British islands to the depth of four- 

 teen feet. As this water supplies the trade-winds with vapom', it 

 therefore becomes salter, and as it becomes Salter, it becomes 

 heavier ; and therefore we may infer that the forces of aggi^egation 

 among its particles are increased. 



106. Whatever be the cause that enables these trade-wind waters 

 Amount of salt left to remain on the surface, whether it be from the 

 by evaporation. £^^j. j^^^^ stated, and in consequence of which the 

 waters of the Gulf Stream are held together in their chamiel ; or 

 whether it be from the fact that the expansion from the heat of the 

 torrid zone is sufficient to compensate for this increased saltnes3 ; or 

 whether it be from the low temperature and high satm-ation of the 



