NARRATIVE OF THE CRUISE. 85 



disadvantages in this form, namely, the indications are not continuous, but by jerks, 

 depending on the size of the mercury drops, and they require to be constantly set, the 

 maximum at a higher and the minimum at a lower temperature than the one to be 

 observed ; and they also require constant comparison with a standard. They are, there- 

 fore, not suitable for use where many observations have to be made expeditiously. 



In the year 1782, Six * published a description of the combined maximum and minimum 

 thermometer which bears his name, and which has since continued to assert its place 

 among meteorological instruments as, perhaps, the best self-registering thermometer for 

 sea temperature observations. The instrument is too well known to require particular 

 description. It may, however, be noted that Six himself did not use a hair for a 

 spring to keep his indices from falling down, but a fine glass thread soldered to the 

 top of the index, and sticking up in a direction very slightly inclined to that of the length 

 of the index, so that it pressed gently against the sides of the tube. The advantage 

 of the glass over the hair is that it does not lose its elasticity ; but, on the other hand, 

 the index takes up more room, and requires a thermometer with a longer stem. 



Maximum and minimum thermometers such as Cavendish's and Six's, when used for 

 deep-sea exploration, show only the maximum and minimum temperatures to which they 

 have been exposed in any one excursion, and a single observation with such a thermo- 

 meter does not give with certainty the temperature of the water at the depth to 

 which it has been sunk. Hence, if it were possible for the temperature of a sea or lake 

 to vary in any conceivable way with the depth, these instruments would be valueless. 

 There is, however, no justification for this assumption ; it is known, on the contrary, 

 that in all seas where the surface is not exposed to a freezing temperature, the tempera- 

 ture of the water, as a rule, diminishes as the depth increases ; and therefore that the 

 minimum temperature, as shown by the self-registering thermometer, is, in fact, the 

 temperature at the greatest depth attained by the instrument. Hence, in such 

 cases, this instrument is to be relied on, and more especially when series of temperatures 

 are taken — that is, when the temperatures at different depths in the same locality are 

 taken, so that the evidence of the decrease of temperature with increase of depth is 

 rendered as strong as possible. In order to render an account of the state of the 

 sea as regards temperature, it is absolutely necessary to have such serial observations ; 

 hence, for such investigations, the maximum and minimum thermometer is not only 

 perfectly trustworthy, but a most valuable and, indeed, indispensable instrument, for it 

 has the great advantage that, as it is in the strictest sense se?/-registering, any number 

 can be attached to the same line, and so at one haul the temperature can be observed at 

 a number of different depths. 



The instrument used for almost all the observations made on board the Challenger, 

 was Six's thermometer with a double bulb, of the pattern made by Mr. Casella for deep-sea 



1 Phil. Trans., vol. lxxii. p. 72, 1782. 



