636 TEMPERATUEE OF THE DEEP SEA. 



thermometers from the effects of pressure, which was ably carried out by 

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

 the Admiralty, and resulted in the construction of an almost perfect 

 instrument for recording the temperature at great depths.* 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, was reduced to a minimum ; and the liability to accident, 

 iilmost inseparable from instruments containing large quantities of this 

 heavy substance, was thus greatly reduced. 



" The cruise of H.M.S. Challenger afforded ample opportunities for 

 testing the capabilities of the Miller- Casella Thermometer. It resisted all 

 the pressures to which it was exposed down to a depth of about 4,000 

 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 4 to 5 

 miles are exceptional, such accidents will be of rare occurrence. Under 

 a pressure of three tons, equivalent to a depth of 3 miles, the error 

 amounts to less than 1° C. (2° F.), whilst that of the unprotected ther- 

 mometers, previously in use, sometimes exceeded 10° C. (18° F.) for the 

 same depth ; and a comparison of the data furnished by the Miller- 

 Casella Thermometer with the corresponding Temperature Curves shows 

 that the mean error of all the deep-sea observations made on board 

 H.M.S. Challenger, not much under 10,000 in number, is probably less 

 than 0-5° C. 



" The Thermometers, before being sent out, are subjected to pressures 

 varying from one to four tons to the square inch, in a hydraulic press 

 especially designed for this purpose by Mr. Casella, and the amount of 

 Arror ascertained for each instrument. When in actual use, they are 

 enclosed in a copper cylinder, perforated at both ends, to allow free ingress 

 a ad egress of the water. Several Thermometers may be attached to 

 uhe same sounding-line, whilst it is being paid out, at distances of 

 5, 10, 25, 50, or 100 fathoms as required ; and it is found that an im- 

 mersion of from five to ten minutes is sufficient to secure the desired 

 record of temperature. 



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

 her cruise round the world, were obtained, and the true Temperature of 

 the sea determined by means of this instrument, now known as the 

 Miller-Casella Thermometer, a description of it in these pages may not be 

 out of place. 



* In experiments with the hydraulic machine, constructed to test the effect of 

 pressure on the thermometers employed, a standard registering thermometer, shielded 

 in a stout glass cylinder, was placed in the water together with one of the deep-sea 

 thermometers, and the water was compressed to an extent equal in pressure to about 

 3 miles of depth in the sea. All previous suspicions of errors were fully confirmed, m 

 the pressure on the bulb was found to cause an error of 12° to 13" P. at this depth. 

 Notwithstanding the great pressure to which these instruments had been subjected, all 

 of them, without exception, recovered their original scale-readings as soon as th« 

 pressure was removed. 



