98 On Similarities in the 



This diagram shows well the distinguishing features of an 

 equatorial ocean in the warm season of the year. In the cooler 

 season the surface temperature is below 80, and the temperature 

 gradients in the layers near the surface are gentler. At this 

 season, with the sun nearly vertical over the equator, and a 

 thick layer of water of temperature over 80 F. on the surface, 

 the gradients are very steep. This was found both by the 

 "Challenger" and the "Buccaneer," in which ship several 

 of the "Challenger's" temperature stations were repeated. 

 Thus station 43 of "Buccaneer" was made on 7th March, 

 1886, in the same position as No. 346 of "Challenger" on 

 6th April, 1876, namely, lat. 2 42' S., long. 14 43' W. The 

 "Challenger" found a higher surface temperature, 82-7 against 

 81-7 ; but the layer of water above 80 F. was thicker in March 

 1886 than in April 1876. The following temperatures were 

 observed : 



Depth . . . . Surface 10 20 25 30 50 



_, ("Challenger" 82-7 82-0 78-1 71-5 62-2 56-5 F. 



im P- "(''Buccaneer" 81-7 81-2 81-0 65-2 61-8 57'3F. 



It is remarkable that the "Buccaneer" found a fall of 

 16 F. in 5 fathoms between 20 and 25 fathoms ; the steepest 

 gradient found by the "Challenger" was about 9-3 in 5 fathoms 

 between 25 and 30 fathoms. Similar gradients and even 

 steeper ones are found in our fresh-water lakes about mid- 

 summer, when the surface heating is proceeding most rapidly. 

 In July of this year (1886) a fall of 5 in one fathom was observed 

 in Loch Lomond at a depth of 12 fathoms 1 . 



In the diagram the thick lines are those of equal density at 

 constant temperature (60 F.). The line of 1-027 shows the 

 penetration of dense trade-wind water, and the effect of high 

 surface density in facilitating the propagation of heat down- 

 wards is well shown by the dipping of the isotherms. The 

 surface outcrops of the 1-026 line to the north of the line in- 

 dicate the average position of the equatorial freshet. On this 

 line neither the "Challenger" nor the "Buccaneer" experienced 

 any sensible easterly current, and it is well known that the 

 Guinea current experiences marked variations with the season. 



In the "Buccaneer" it was the custom every day at I P.M. 

 1 See Paper No. 21, p 312. 



