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HISTORY OF THE SEA. 



of silver are soldered with silver solder to an intermediate plate 

 of gold, and this compound ribbon is coiled round a central 

 axis of brass, with the silver inside. Silver is the most expan- 

 sible of the metals under the influence of heat, and platinum 

 nearly the least Gold holds an intermediate place, and its 

 intervention between the platinum and silver moderates the 

 strain and prevents the coil from cracking. The lower end of 

 the coil is fixed to the brazen axis, while the upper end is fast- 

 ened to the base of a short cylinder. Any variation of tem- 

 perature causes the coil to wind or unwind, and its motion 

 rotates the axial stem. This motion is increased by multiply- 

 ing wheels, and is registered upon the dial of the instrument by 

 an index, which pushes before it a registering hand, moving 

 with sufficient friction to retain its place, when pushed for- 

 ward. The instrument is graduated by experiment. The 

 brass and silver parts are thickly gilt by the electrotype pro- 

 cess, so as to prevent their being acted upon by the salt water. 



The box in which the instrument is protected is open to 

 admit the free passage of the water. This instrument seems 

 to answer very well for moderate depths. Up to six hundred 

 fathoms its error does not exceed a half degree, centigrade ; at 

 1,500 fathoms it rises however to five degrees, quite as much 

 as an unprotected Six thermometer, and the error is not so con- 

 stant. Instruments which depend for their accuracy upon the 

 ■working of metal machinery cannot be depended upon when 

 subjected to the great pressure of deep soundings 



For taking bottom temperatures at great depths, two or 

 more of the thermometers are lashed to the sounding line at a 

 little distance from each other, a few feet above the sounding 

 instrument. The lead is rapidly run down, and after the bot- 

 tom is reached an interval of five or ten minutes is allowed 

 before hauling in. In taking serial temperature soundings, 

 which are to determine the temperature at certain intervals of 

 depth the thermometers are lashed to an ordinary deep sea 



