258 THE PHYSICAL GEOGRAPHY OF THE SEA. 



its circulation would be torpid, and its bosom lack animation. 

 This under current may be freighted with heat to temper some 

 hyperborean region or to soften some extra-tropical climate, for 

 we know that such is among the effects of marine currents. At 

 starting, it might have been, if you please, so loaded with solid 

 matter that, though its temperature were 90°, yet, by reason of 

 the quantity of such matter held in solution, its specific gravity 

 might have been greater even than that of extra-tropical sea water 

 generally at 28°. Notwithstanding this, it may be brought into 

 contact, by the way, with those kinds and quantities of marine or- 

 ganisms that shall- abstract solid matter enough to reduce its spe- 

 cific gravity, and, instead of leaving it greater than common sea 

 water at 28°, make it less than common sea water at 40 ; conse- 

 quently, in such a case, this warm sea water, when it comes to the 

 cold latitudes, would be brought to the surface through the in- 

 strumentality of shell-fish, and various other tribes that dwell far 

 down in the depths of the ocean. Thus we perceive that these 

 creatures, though they are regarded as beings so low in the scale 

 of creation, may nevertheless be regarded as agents of much im- 

 portance in the terrestrial economy ; for we now comprehend how 

 they are capable of spreading over certain parts of the ocean those 

 benign mantles of warmth which temper the winds, and modify, 

 more or less, all the marine climates of the earth. 



489. The makers of nice astronomical instruments, when they 

 The regulators of the li^vc put the different parts of their machinery to- 

 ^®*' gether, and set it to work, find, as in the chronom- 



eter, for instance, that it is subject in its performance to many ir- 

 regularities and imperfections ; that in one state of things there is 

 expansion, and in another state contraction among cogs, springs, 

 and wheels, with an increase or diminution of rate. This defect 

 the makers have sought to overcome ; and, with a beautiful dis- 

 play of ingenuity, they have attached to the works of the instru- 

 ment a contrivance which has had the effect of correcting these 

 irregularities by counteracting the tendency of the instrument to 

 change its performance with the changing influences of tempera- 

 ture. This contrivance is called a compensation ; and a chronom- 

 eter or clock that is well regulated and properly compensated 

 will perform its ofiice with certainty, and preserve its rate under 

 all the vicissitudes of heat and cold to which it may be exposed. 



