192 Inslrudiom for the Scientific Expedition 



is the view taken by M. Arago in his daborate instructions for the 

 voyage of the Bonite, and it would appear undoubtedly mtre just than 

 that which attributes them wholly to the friction and pressure of the 

 winds. Nevertheless it must not be forgotten that there is an essen- 

 tial difference in the modes of action in the two cases. The sun's 

 heat is effective in heating the air mainly from below, where it is in 

 contact with the earth or water which absorb the rays and commu- 

 nicate them to the air above. In the sea the case is otherwise. 

 The sun's rays are totally absorbed at the surface, and no ray reaches 

 the bottom of any sea deserving the name. No deep stratum of 

 water, therefore, can be permanently maintained by the sun's direct 

 heat at a temperature greatly above what it would have independ- 

 ently of its direct action. Hence the motive power in a system of 

 currents so originating must be sought, not in the ascensional force 

 at the equator, but in a descensional one in the polar regions, or rather 

 in that one polar region in which winter prevails. The order of the 

 phenomena then is precisely the reverse of what obtains in the 

 atmosphere ; moreover the seat of the efficient agency is not only 

 much less extensive than in the case of the atmospheric currents, but 

 also subject to a semiannual shifting from one to the other extremity 

 of the earth's axis, both which causes must tend greatly to diminish 

 the average energy of the effect. 



Practically speaking, the question resolves itself into one of fact, 

 which observation only can decide. Is there in the whole column 

 of water between the surface of the ocean and its bed at the poles, 

 as compared with a column of equal depth at the equator and in free 

 communication with it, a descensional power or not ? and what is its 

 amount 1 These questions can only be resolved by observations of the 

 temperature and saltness of the sea, at various and considerable 

 depths, in different latitudes, and under a great variety of local cir- 

 cumstances. The procuring such observations, and the preservation 

 of specimens of the water, or the determination on the spot of their 

 specific gravities, will afford a useful occupation in calms, and may 

 be recommended as well worthy of attention. Theoretically speaking, 

 the subject is more complicated than at first appears, since it cannot 

 but be that some considerable portion of solar heat absorbed by the 

 equatorial continents, — in place of finding its way out of the earth by 

 radiation at the poles, in the mode of subterraneous communication 

 suggested by Fourrier, — must escape through the bed of the ocean 

 into its waters, and so be carried into their circulation. 



Opportunities for determining the temperature of the ocean at 

 great depths must of course be rare ; but at moderate depths it can 

 always be done with comparatively little trouble, and we would, 

 therefore, suggest the propriety of making observations of this ele- 

 ment at two moderate and constant depths (say 150 and 300 

 fathoms), by the aid of a self- registering thermometer attached to a 

 sounding line whenever the ship's way shall be such as to allow their 

 being made with precision. 



