DEEP-SEA THEEMOMETERS. 



637 



" The Milleb-Casella Thermometer, as will be seen from the accom- 

 panying figures is designed to register the maximum and minimuiQ tem- 

 perature of the water to which it is exposed during its descent from the 



the surface of the sea to the bottom. For 

 this purpose, the glass tube is bent in 

 the shape of the letter U, each arm of 

 the tube terminating in a bulb. The 

 larger bulb, A, is surrounded by another 

 bulb, B, and about 

 three-fourths of the 

 space between the two 

 bulbs is filled with 

 alcohol. It is by the 

 addition of this outer 

 bulb, B, that the pro- 

 tection of the instru- 

 ment from the effects 

 of pressure is secured. 



"On immersion, the 

 outer bulb receives the 

 pressure of the water, 

 and forces the enclosed 

 alcohol into the por- 

 tion of the intervening 

 space previously not 

 occupied by this liquid, 

 thus relieving the in- 

 ner bulb, A. The 

 latter is completely 

 filled with a mixture 

 of creosote and alcohol, 

 which rests upon the 

 mercury contained in 

 the bend of the tube, 

 and also fills up the other arm and part of the bulb C. The upper part 

 of bulb C is occupied by air, introduced, with the help of a freezing 

 mixture, at a very low temperature, in order to increase its elastic force. 

 It thus acts as a sort of elastic cushion, intended to overcome the 

 weight and friction of the mercury in the tube, and to assist the mercury 

 in following the mixture when contracting under the influence of cold. 



" The indications of the thermometer depend upon the expansion by 

 heat and contraction by cold of the mixture contained in bulb A. When 

 expanding, the mercury is forced down in the arm attached to this bulb 

 and rises in the other arm towards bulb C ; when contracting, the mercury 

 falls on the side of bulb C, and rises towards bulb A. Two steel indices, 

 a, a, covered with glass, mark the maximum height which the mercury 

 has reached in either arm, and a hair attached to each index produces 

 the friction necessary to retain them at the level to which they have 

 been raised. Before the thermometer is lowered into the sea, the 



Miller- Casella 



Therinomeler, 



in case. 



