184 Prof. Hennessy on Terrestrial Climate as 



If, as in the existing oceans, this water be salt, the inequali- 

 ties of temperature, producing inequalities of evaporation, will 

 also produce diversities in the density of the water in different 

 regions, and thus additional energy will be imparted to the pro- 

 cess of circulation. The Salter and heavier surface water will 

 tend to sink into the colder hquid which lies beneath, and which 

 will naturally tend to take its place, by ascending upwards*. 

 The process of evaporation would cool the surface of the water ; 

 but, unlike that of radiation, it is not altogether a losing pro- 

 cess so far as the entire surface of the earth is considered ; for 

 it is sooner or later followed by condensation, whei-eby the greater 

 part of the absorbed heat is again returned. When a piece of 

 land or water parts with its heat by radiation into space, that 

 warmth can never be restored to any part of the earth's surface ; 

 but whatever heat the water loses by evaporation, becomes la- 

 tent in the vapour so produced, and is ultimately transferred 

 by condensation to some other part of the globe; and hence 

 evaporation does not constitute an agent in causing a dimi- 

 nution of general terrestrial temperature. Let us now suppose 

 a sheet of water at the equator nearly surrounded by fixed 

 boundaries, so as to form a species of immense lagoon. Its 

 temperature, from the causes here referred to, will rapidly aug- 

 ment. The heat which it has acquired during the day will 

 have penetrated so deeply as to be incapable of being radiated 

 backwards into space, during the night, with the same facility 

 as on the surface of a sandy plain or from the summits of a 

 mass of vegetation. Its temperature should thus continue to 

 accumulate up to a certain limit imposed by the conditions of 

 evaporation ; and it might ultimately attain a mean temperature 

 superior to any which is now met at the surface of intertropical 

 seas. 



3. These views are strikingly illustrated by the ph?enomena 

 accompanying the origin of the Gulf-stream. The mass of 

 water which nishes into the Gulf of Mexico, along the southern 

 shores of the Caribbean Sea, has already acquired a certain ele- 

 vated temperature from the action of sunshine in the southern tor- 

 rid zone in its passage from Cape St. Roque. In moving around 

 the Caribbean Sea and the Mexican Gulf, these waters still 

 continue under the influence of a tropical sun, and are constantly 

 increasing in temperature. The islands and coasts which they 

 happen to bathe, have no part in directly promoting this augmen- 

 tation. On looking over the isothermal chart of the Caribbean 

 Sea and Gulf of Mexico, prepared by M. Charles Devillef, it 

 becomes manifest that in general the temperature decreases in 



* See Maury, 'Physical Geography of the Sea,' p. 160. 



t Anvuaire de la Societe Met^orologique de la France, torn i. p. 160. 



