260 R. HITCHCOCK ON THE COAST SURVEY AND THE 



radiolaria are found at immense depths, and it has not yet been 

 explained how it is that solution takes place in some localities and 

 not in others. 



The sinker used for depths of 1,000 fathoms weighs 341bs. ; for 

 greater depths a GOlb. weight is used, and is quite sufficient, and 

 seldom exceeded, except where there were very rapid currents. The 

 341b. weight is always recovered, but in deep casts it is usual to 

 leave the sinker at the bottom to relieve the wire of the strain of 

 hauling it up. 



For collecting small samples of the bottom a sounding rod is used, 

 which consists of a brass cylinder about a foot long, with a valve at 

 the bottom, opening upwards and closed by a spring. When this 

 strikes a sandy or soft bottom some of the material passes the valve, 

 and is thus secured. The sinker is a spherical cast-iron shot, with 

 a hole bored through the middle. The sounding rod fits loosely in 

 the hole, and the shot is sustained on the rod by a wire sling, which 

 extends up to the hook of the detacher. The detacher is a simple 

 contrivance' for releasing the shot when the bottom is reached. 



The first efforts to obtain the temperature of the sea at great 

 depths gave absolutely no results, because the thermometer bulbs 

 were so compressed by the weight of water, which sometimes reaches 

 several tons to the inch, that the mercury was driven far up the 

 stem. 



Messrs. Negretti and Zambra first produced a thermometer suffi- 

 ciently strong to resist this pressure by forming it with two bulbs, 

 one within the other, the outer one being partially filled with 

 mercury and exhausted of air. After this Cassella applied the same 

 protecting principle to a " Sixe's " thermometer, but used alcohol 

 instead of mercury in the outer bulb. 



For a long time the Miller Cassella thermometer, as it was 

 termed, was the only one available for taking serial temperatures. 

 It was made on Sixe's pattern in order to register the maximum 

 and minimum temperatures, but it was not altogether satisfactory, 

 being rather fragile and liable to have the indices displaced. 



Another form was therefore devised by Messrs. Negretti and 

 Zambra. This has a bend just above the bulb, and by inverting 

 the instrument the column of mercury parts at this bend. 



This instrument is now used almost exclusively. It has only to 

 be placed in a case which can be turned over end for end, this being 

 sent down and inverted at any particular depth. When it is again 



