of Edinburgh, Session 1878 - 79 . 
79 
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 thermo- 
meter 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 thermometer 
does not give us with certainty the temperature of the water at the 
depth to which it has been sunk. Hence, if we had a right to 
assume that the temperature of a sea or lake might vary in any con- 
ceivable way with the depth, these instruments would be valueless. 
We have, however, no right to make this assumption ; we know, on 
the contrary, that in all seas whose surface is not exposed to a 
freezing temperature, the temperature of the water will as a rule 
diminish as the depth increases ; that, therefore, the minimum tem- 
perature, as shown by the self-registering thermometer, will, in fact, 
be the temperature at the greatest depth attained by this thermo- 
meter. 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 any lake or 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, indis- 
pensable instrument, for it has the great advantage that, as it is in 
the strictest sense ^/-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. 
For isolated observations the thermometers just described are not 
so satisfactory, and a very great amount of ingenuity has been dis- 
played in the invention of machines for registering the actual 
temperature of the water at any depth independently of that of the 
water above it. Hone of the instruments devised for this purpose 
have been strictly ^/-registering ; they have all required some 
assistance from the observer, who, by various forms of mechanical 
appliance, brings about a catastrophe which leaves its mark on the 
