1869.] 



on the Scientific Exploration of the Deep Sea. 



411 



several others at a depth of above two miles), they have been found to be 

 in as good order as when they were first sent out by Mr. Casella. While 

 this most satisfactory result is partly due to the careful handling of the 

 apparatus by Capt. Calver, it is mainly attributable on the one hand to 

 the excellence of the principle on which the Thermometers are constructed, 

 and on the other to the admirable workmanship of Mr. Casella ; for the 

 records of previous Temperature- Soundings show that the fracture of 

 the bulbs of unprotected Thermometers at great depths was a very 

 common occurrence, whilst the record of observations made in the 

 ' Lightning' Expedition* shows that the indications of Thermometers of 

 less perfect construction often show a considerable discrepancy. 



16. In concluding this accouut of the behaviour of the protected Miller- 

 Casella Thermometers under the most trying conditions, it may be added 

 that wherever the localities of the Temperature-Soundings taken with these 

 instruments during the Third cruise of the ' Porcupine' were the same 

 (or nearly so) with those of the Temperature-Soundings taken in the 

 1 Lightning' Expedition, their correspondence proved to be very close, 

 when the proper correction for the depths at which they were taken was 

 applied to the latter (§ 95). Thus the differences of temperature between 

 the Warm and the Cold Areas indicated by those observations f remained 

 the same, although the Temperatures recorded by the " unprotected " Ther- 

 mometers required to be reduced by from 2° to 3° to show the actual tempe- 

 ratures, — a recorded temperature of 46° at 650 fathoms in the Warm Area 

 indicating a real temperature of 43°, while a recorded temperature of 32° at 

 550 fathoms in the Cold Area indicated a real temperature of about 29 - 8°. 



17. As it was considered expedient by the Committee (p. 402) that a trial 

 should be given to Mr. Siemens' s apparatus for the determination of deep-sea 

 temperatures, this apparatus (which he terms a Differential Thermometer) 

 was fitted on board the ' Porcupine,' and provided with 1000 fathoms of a 

 small cable about the size of the ordinary Sounding-line, which contained 

 the two insulated wires necessary for the establishment of the two circuits 

 to be brought into comparison. The indications of this instrument depend 

 upon the equalization of two currents transmitted through resistance-coils 

 of fine platinum wire ; one of these coils being sent down at the end of the 

 sounding-cable, whilst the other is immersed in a vessel on deck, the water 

 in which can be gradually lowered in temperature by the addition of ice 

 or the use of a freezing-mixture. When the equalization of the currents 

 is shown by the galvanometer, the temperature of the water in the vessel 

 on deck (indicated by an ordinary thermometer) will represent that of the 

 stratum of the sea beneath, in which the resistance-coil is immersed at the 

 time. — Nothing can be more perfect than the working of this apparatus 

 when the Galvanometer rests on a fixed plane surface ; and its accuracy 

 and delicacy were satisfactorily proved by experiments carried on not 



* Proceedings of the Royal Society, Dec. 17, 1868, p. 172, notes. 

 t Ibid. p. 188. 



