20 THREE CRUISES OF THE " BLAKE." 



the water surrounding it, and a second period of five minutes 

 for adjusting the temperature of the comparison-coil on deck. 

 Allowing five minutes more for lowering the cable, fifteen min- 

 utes sufiiced to complete a deep-sea observation. 



The instrument was sufficiently tested by Commander Bart- 

 lett, during the season of 1881, to show its entire practicabiUty. 

 The accompanying table is taken from his Report to Pro- 

 fessor J. E. Hilgard, the superintendent of the United States 

 Coast Survey : — 



Depth in Fathoms. Siemens. Miller- Casella. 



Surface . 77^ 77^ 



5 fathoms 764- 754 



10 " 75^ 69 



15 « 661 631 ;»! 



20 « 58 57 



30 " 511 51L 



50 " 544 52I 



75 " 53^ 53^ 



100 " 51 44 



125 « 48J 



150 « 46^ 46 



200 " 431 43J 



300 " 401 40| 



400 « 40 39| 



500 « 39^ 39 



600 " 38f 38| 



700 " 381 38^ 



800 " 381 38^ 



Much yet remains to be done before we can ascertain accu- 

 rately not only the force, but also the direction of the currents, 

 at different depths, from the surface to the bottom ; the floats 

 usually employed to make these observations are rather primi- 

 tive instruments. Here also we must look to electricity for the 

 means of making simultaneous or continuous observations. 



The specific gravity of the sea-water was determined on' the 

 " Blake " by means of an apparatus devised by Professor J. E. 

 Hilgard (Fig. 16) of the Coast Survey. As the differences of 

 density are very small, the instrument must be one of great 

 delicacy ; it is constructed upon the same principle as that of 



1 According to Commander Bartlett, 30 fathoms, which was not detected by 

 the Siemens apparatus recorded the the Miller-Casella, indicating only max- 

 presence of a warm belt between 10 and ima and minima. 



