122 Prof. W. Thomson on the Depths of the Sea. 



found no inconvenience at all from the pressure, except in one 

 particular. The strong tarred hemp rope which we used be- 

 longed to the upper world, and, like all such terrestrial fabrics, 

 it contained a large quantity of air. Down in the depths every 

 particle of the air was squeezed out, and the fibres of the hemp 

 and the tar were crushed together, so that the rope looked and 

 cut almost like a stick of liquorice. I fear the rope became 

 rather brittle ; for it snapped once or twice without apparent 

 cause, and we lost our dredges. This may turn out to be a 

 serious difficulty in the way of dredging in much greater 

 depths. 



Temperature. — There has been up to the present time a 

 strange misconception as to the temperature of the ocean — a 

 misconception all the more singular as it is a point easy of 

 aj)proximate determination, and to which a good deal of at- 

 tention has been directed. In all the leading text-books on 

 physical geography we have the reiterated statement that at 

 a certain depth the ocean has a uniform temperature of 39° F., 

 that the ocean is, therefore, divided into three regions, bounded 

 by the two isotherms of 39° F., that north and south of these 

 lines the mean temperature of the surface is lower than that of 

 the depths, while in the zone between them it is higher. Had 

 the sea been fresh, it would have been perfectly intelligible 

 that the water beyond the influence of currents and of direct 

 solar heat should have maintained the temperature of its point 

 of greatest density ; but it has long been well known, from 

 the experiments of M. Despretz and others, that sea-water 

 contracts steadily down to its freezing-point, which is about 

 28° F. when agitated, and as low as 25° F. when perfectly 

 still. 



Though I had often wondered what could be the cause, I 

 believed in this permanent temperature of the sea thoroughly, 

 and even suggested the particular course for our cruise, because 

 it nearly coincided with the isotherm of 40° F., expecting that 

 we should be able, within a few hundred feet of the surface, 

 to eliminate the question of heat entirely from our calculations. 

 To our very great surprise, the thermometers, two of which 

 were sent down on the lead-line, the day after we left Storno- 

 way, to a depth of 500 fathoms, registered a minimum tem- 

 perature of 49°, ten degrees above the " permanent point." We 

 were at first inclined to mistrust the observation ; but we took 

 the same temperature at nearly the same spot on our return, 

 when we were quite prepared to recognize it as the almost 

 constant temperature of the warm or Gulf-stream area of the 

 region. Some days later, on leaving Thorshaven and pro- 

 ceeding south-eastwards, we sounded and took temperatures 



