32 Temperature of the Ocean. 



a happy coincidence, the desired improvement was effected at 

 the moment when a second expedition was being prepared. In 

 April, 1869, previous to the departure of H.M.S. "Porcupine" 

 on her first cruise, at a meeting of the Deep-sea Committee of 

 the Royal Society of London, Dr. W. A. Miller, V.P.R.S., 

 suggested a simple expedient for protecting thermometers from 

 the effects of pressure, which, ably carried out by Mr. L. P. 

 Casella, F.R.A.S., the eminent scientific instrument maker to 

 the Admiralty, resulted in the construction of an almost perfect 

 instrument for recording the temperature at great (depths. Since 

 all the observations made on board H.M.S. "Challenger," 

 during her cruise round the world, were obtained with the help 

 of this instrument, now known as the Miller-Casella ther- 

 mometer, a description of it in these pages may not be out of 

 place. 



The Miller-Casella Thermometer. — It will be seen from 

 the accompanying figure that this thermometer is designed to 

 register the maximum and minimum temperature of the water 

 to which it is exposed during its descent from the surface of the 

 sea to the bottom. For this purpose the glass tube is bent in 

 the shape of (J' ^^ch 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 

 protection of the instrument 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 portion of 

 the intervening space previously not occupied by this liquid, 

 thus relieving the inner bulb, A. The latter is completely filled 

 with a mixture of creosote, alcohol, and water, 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 



