70 THE BOTTOM OF THE SEA. 



Perhaps the best methoil discovered up to the pn^- 

 sent time consists in enclosing one of Walferdin's 

 metastatic thermometers in a strong box of wrought- 

 iron or copper, noting on the return of the instrument 

 the lowest temperature that it has marked. Some- 

 times, however, the box will be found crushed. 



7. Diminished Temperature of the Sea in proportion to the Depth 

 — Irregularities introduced in this law by tlie iafluencD of Sub- 

 mariue Currents — Temperature at the Bottom of the Oeeaa 

 constant and uniform— Principal causes of Submarine Currents. 



The temperature of the atmosphere diminishes in 

 the degree that we ascend above the level of the sea ; 

 that of the sea generally diminishes in the degree 

 that we descend below its surface. It varies but little 

 from day to night, and even from season to season. 

 At no great depth it ceases to vary at all. 



The surface is hottest at the equator; it is frozen 

 at the poles. Between these extreme latitudes there 

 is a succession of diminishing temperatures, but they 

 are far from decreasing in regular gradation from the 

 equator to the poles. The water is influenced by 

 marine currents, which have the effect of masking the 

 otherwise regular law of decrease. 



The law which varies the temperature according to 

 the depth is also complicated by accidental causes. 

 Often marine currents flow one above another — the 



