in the Twenty Years before 1895 73 



The average temperature at which all the "Challenger" 

 surface observations were made is 20-94 C. If the Antarctic 

 cruise is excluded, the average in Atlantic and Pacific taken 

 together is 22-87 C., the average in the Pacific alone being 

 23-97 C., and in the Atlantic alone 21-32 C. In exploring 

 equatorial waters very few observations would be made at 

 a temperature below 25 C., and, except under special and 

 not particularly agreeable circumstances, very few above 

 29 C., and between the tropics they would all fall above 

 20 C. Looking to the much greater extent of the waters repre- 

 sented by the Atlantic and Pacific part of the cruise, it might 

 seem to be more convenient to take the temperature resulting 

 from their consideration than that found when the Antarctic 

 is included ; but it must not be forgotten that, though smaller 

 in extent , the cold regions of the surface of the ocean are of 

 great interest from a physical and chemical point of view, 

 and it would seem that for a common temperature to which 

 to reduce all densities, 20 C. would be the most convenient. 

 The large number of densities already existing which are 

 reduced to the constant temperature 15-56 C. are transformed 



-ubtracting o-oono; and these which are expressed as 

 specific gravities at 17-5 C., referred to that of distilled water 

 of the same temperature as unity, are transformed by sub- 

 tracting 0-00189 for average ocean water. These corrections, 

 applied to the reduction tables now in use, would at once 

 fit them equal Iv well for a reduction to a common temperature 

 of 20 C., and with these tables the density of the water in situ 



dso at once found. 



To recapitulate: In determining the density of the sea- 

 water in an expedition only the absolutt wt i-lit hydrometer 

 should be used. The samples of water should be stored in 

 laboratory win -re the observations are going to be made, 



thev should have seiiMMv t he same temperature as the 

 \hile the observations are being made. If the m 



iiip i^ at anv time ton violent for it to be convenient 



itiooi, then .1 ufficient supply of bottle^ 



.Id he .it hand to keep the samples until the motion becomes 



without mi* with the ...lie, tion of other samples. 



