Messrs. Negretti and Zambra on a Deep- sea Thermometer. 307 



thermometer, as regards the protection of the bulb and its non- 

 liability to be affected by pressure, is all that can be desired ; but 

 unfortunately the only thermometer available for the purpose of 

 registering temperature and bringing those indications to the sur- 

 face is that which is commonly known as the Six's thermometer 

 — an instrument acting by means of alcohol and mercury, and 

 haying movable indices with delicate springs of human hair tied to 

 them. This form of instrument registers both maximum and mi- 

 nimum temperatures ; and as an ordinary out-door thermometer it 

 is very useful; but it is unsatisfactory for scientific purposes, 

 and for the object for which it is now used (viz. the determination 

 of deep-sea temperatures) it leaves much to be desired. Thus 

 the alcohol and mercury are liable to get mixed in travelling, or 

 even by merely holding the instrument in a horizontal position ; 

 the indices also are liable either to slip if too free, or to stick if 

 too tight. A sudden jerk or concussion will also cause the in- 

 strument to give erroneous readings by lowering the indices, if 

 the blow be downwards, or by raising them, if the blow be up- 

 wards. Besides these drawbacks, the Six's thermometer causes the 

 observer additional anxiety on the score of inaccuracy ; for, although 

 we get a minimum temperature, we are by no means sure of the 

 point where this minimum lies. Thus Professor Wyville Thomson 

 says ('Depths of the Sea,' p. 139): — " The determination of tem- 

 perature has hitherto rested chiefly upon the registration of mini- 

 mum thermometers. It is obvious that the temperature registered 

 by minimum thermometers sunk to the bottom of the sea, even if 

 their registration were unaffected by the pressure, would only give 

 the lowest temperature reached somewhere between top and bottom, 

 not necessarily at the bottom itself. The temperatures at various 

 depths might indeed (provided they nowhere increased on going 

 deeper) be determined by a series of minimum thermometers placed 

 at different distances along the line, though this would involve 

 considerable difficulties. Still, the liability of the index to slip, 

 and the probability that the indication of the thermometers would 

 be affected by the great pressure to which they were exposed, ren- 

 dered it very desirable to control their indications by an indepen- 

 dent method." Again, at page 299, we find : — " I ought to men- 

 tion that in taking the bottom temperature with the Six's thermo- 

 meter the instrument simply indicates the lowest temperature to 

 which it has been subjected; so that if the bottom water were 

 warmer than any other stratum through which the thermometer 

 had passed, the observations would be erroneous." Undoubtedly 

 this would be the case in extreme latitudes, or in any spot where 

 the temperature of the air is colder than that of the ocean. 

 Certainly the instrument might be warmed previous to lowering ; 

 but if the coldest water should be on the surface, no reading, to be 

 depended upon, could be obtained. 



It was on reading these passages in the book above referred to 

 that it became a matter of serious consideration with us whether a 



X2 



