The Miller-Casella Thermometer. 33 



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

 density. It acts as a sort of elastic cushion intended to over- 

 come the friction of the liquids in the tubes, 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 metal 

 indices, a a, mark the maximum height which the mercury has 

 reached in either arm, and a hair attached to each index pro- 

 duces the friction necessary to retain them at the level to which 

 they have been raised. Before the thermometer is lowered into 

 the sea, the indices are brought down upon the mercury by 

 passing a magnet along the tube. One of the essential qualities 

 of an instrument intended for use on long voyages is that it 

 should be portable — a quality especially realised by Mr. Casella 

 in the construction of his thermometer. By contracting the 

 bore of the glass tube as much as possible, the quantity of the 

 liquids, particularly of the mercury, has been reduced to a 

 minimum ; and the liability to accident, almost inseparable from 

 instruments containing large quantities of this heavy substance, 

 has thus been greatly reduced. 



The cruise of H.M.S. " Challenger" afforded ample oppor- 

 tunities for testing the capabilities of the Miller-Casella ther- 

 mometer. It resisted all the pressures to which it was exposed 

 down to a depth of about 4000 fathoms, when some of the 

 instruments were found to give way under a pressure of four 

 tons to the square inch ; but as depths of from four to five miles 

 are exceptional, such accidents will be of rare occurrence. 

 Under a pressure of three tons, equivalent to a depth of three 

 miles, the error amounts to less than i° C, whilst that of the 



