where 



1 ss k^N 



The quantities Xf and J - + , are depths (feet) of a given iso- 

 therm at the beginning and end of the i t " distance (or time inter- 

 val) along the track, 7^ is the depth difference (feet), and N is 

 the total number of readings in a given series. When the isotherm 

 becomes deeper with distance run, the difference is negative. 



From the speed of the ship and depth differences, approxi- 

 mate slopes can be obtained. At a speed of 6 knots, the ship trav- 

 eled 304 feet in each half-minute interval; therefore, dividing the 

 depth differences by 304 feet gives the slope of the isothermal 

 surface in the direction of the ship's motion. This slope can also 

 be expressed by the angle of which the slope is the tangent. How- 

 ever, it must be pointed out that the slopes measured by this 

 method are influenced by the wave motion, that is, the vertical 

 change of isotherm depth caused by wave motion that takes place 

 in half a minute. Thus, exact slopes cannot be acquired; however, 

 this method yields the best approximation yet obtained of the 

 slopes of isothermal surfaces in the deep ocean. 



At least 1500 observations of isotherm depth were made on 

 each leg of the cruise. The frequency distributions of depth 

 changes and slopes for each selected isotherm on each leg of the 

 cruise are shown in figures 8, 9, and 10. Half-minute depth 

 changes as great as plus or minus 30 feet were observed in the 

 9°C isotherm on the offshore run (fig. 8), corresponding to an 

 angle of 5°38'. Twenty-six per cent of adjacent half -minute read- 

 ings showed depth changes less than 1 foot. However, 50 per cent 

 of the slopes observed on this leg were less than 25 minutes from 

 the horizontal, as indicated by the vertical lines in the figure. 



On the alongshore leg (figs. 9 A and 9 B) nearly all half -minute 

 depth changes were less than 15 feet. For the 14°C isotherm, 50 

 per cent of the depth changes in 304 feet were less than 1. 2 feet, 

 corresponding to a slope of 14 minutes from the horizontal. The 

 9°C isotherm had a somewhat broader distribution of slopes, but 

 the distribution was narrower than for the same isotherm on the 

 offshore leg. Here 50 per cent of the slopes fell within 21 minutes 

 (fig. 9B). 



19 



