726 HISTORY OF THE SEA. 



.importance in physical geography, since an accurate determina- 

 tion of the temperature at different depths is certainly the best, 

 and frequently the only means, for determining the depth, the 

 width, the direction and general path of the warm ocean cur- 

 rents, which are the chief agents in diffusing the equatorial 

 heat; and more especially of those deeper currents of cold 

 water which return from the poles to supply their places, and 

 complete the watery circulation of the globe. The main cause 

 of this want of accurate knowledge of deep sea temperatures 

 is undoubtedly the defective character of the instruments which 

 have been hitherto employed. 



The thermometer which has been generally used for making 

 observations on the temperature of deep water is that known 

 as Six's self-regulating thermometer, inclosed in a strong cop- 

 per case, with valves or apertures above and below, to allow a 

 free passage of the water through the case and over the face 

 of the instrument. This registering, thermometer consists of a 

 glass tube, bent in the form of a U. One arm terminates in a 

 large bulb, entirely filled with a mixture of creosote and water. 

 The bend in the tube contains a column of mercury, and the 

 other arm ends in a small bulb, partly filled with creosote and 

 water, but with a large space empty, or rather filled with the 

 vapor of the mixture and compressed air. A small steel index 

 with a hair tied round it, so as to act like a spring against the 

 side of the tube, and keep the index at any point it may as- 

 sume, lies free in either arm, among the creosote, floating on 

 the mercury. This thermometer gives its indication's only from 

 the expansions and contractions of the liquid in the large full 

 bulb, and consequently is liable to some slight error, from the 

 variations of temperature upon the liquids in other parts of 

 the tube. When the liquid in the large bulb expands, the 

 column of mercury is driven upward toward the half-empty 

 bulb, and the limb of the tube in which it rises is graduated 

 from ^elow, upward, for increasing heat. When the liquid 



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