426 JOURNAL OF THE PLYMOUTH INSTITUTION. 
thirty times the specific heat of mercury, and as water is taken as 
the standard, the specific heat of mercury is 34. The specific heat 
of air is z,; of that of water; hence the energy which will raise 
the temperature of a unit of water 1° of Fahrenheit will raise the 
same unit of air 3234° F. Again, to form vapour at atmospheric 
pressure it requires nine hundred and sixty times as much heat to 
pass into boiling water as it would to raise the same weight of water 
one degree. Tyndall, in his Heat as a Mode of Motion, has most 
vividly put this property of water in the following striking form 
(Art. 240): “The latent heat of aqueous vapour, at the tempera- 
ture of its production in the tropics, is about 1000° F. ; for the 
latent heat augments as the temperature of evaporation descends. 
A pound of water, then, vaporised at the Equator, has absorbed one 
thousand times the quantity of heat which would raise a pound of 
the liquid one degree in temperature. But the quantity of heat 
which would raise a pound of water 1° would raise a pound of 
cast-iron 10°; hence, simply to convert a pound of the water of 
the equatorial ocean into vapour would require a quantity of heat 
sufficient to impart to a pound of cast-iron 10,000° of temperature. 
But the fusing point of cast iron is 2,000° F., therefore for every 
pound of vapour produced a quantity of heat has been expended 
by the sun sufficient to raise five pounds of cast iron to its melting 
point.” From these two causes (specific heat of water and latent 
heat of steam) the temperature of the ocean itself is raised but a 
small amount, whilst it gives to the Return Trades their heating 
power. Then, as the water cools and the vapour condenses, they 
give out just the same amount of heat to cool as was absorbed by 
them in getting heated and changing their form; and it must be 
borne in mind that for every pound of rain that falls enough heat 
has been set free to melt five pounds of cast iron. The ocean thus 
acts as a regulator to the heat supplied by the sun. It absorbs the 
surplus of the Torrid Zone, and of the summer in the Temperate 
Zones, stores it up, and in winter gives it out as the temperature 
of the land falls below the surrounding ocean. The Gulf Stream, 
too, prevents very cold winters in Ireland and South-west England ; 
and although the very existence of such a stream has been ques- 
tioned, I am willing to accept the fact from such authorities as 
Maury, Findlay, Wyville Thomson, and others.* The heat liberated 
* Dr. Carpenter says the Gulf Stream as a distinct current has not been 
traced eastward of 30° W.—WNature, vol. ix. p. 423. 
