86 Proceedings of the Royal Society 
able practical importance, for it is impossible always to protect the 
thermometers when on deck from the direct rays of the sun, which 
would speedily disable the maximum side of the thermometer if its 
range were as limited as that of the minimum one. 
It will be seen from what has been said that there is no one 
instrument which fulfils all conditions required of a perfect deep-sea 
thermometer. It is necessary, therefore, for the investigator to use 
his judgment in the selection of the instrument best suited for the 
particular case before him. In order to be prepared for possibly 
occurring cases, he should be provided with thermometers of (a) 
the Millar-Casella type, with the improvements just described; (b) 
the mercury piezometer ; (c) the Negretti & Zambra inverting 
thermometer. It is well to have several of the first class (a), as any 
number of them can be attached to the line at different depths, and 
thus much time be saved. In my own practice I generally use four 
or five at a time. It is not advisable to exceed this number, as the 
loss in case of accident would be too heavy. Considering the dis- 
tribution of temperature actually found in lakes and seas of warm 
and temperate regions, this is the most generally useful instrument 
when thorough investigation by means of series of observations is 
intended. In the particular and frequently occurring case of an 
enclosed sea containing a large mass of water showing no variation of 
temperature when tested by this instrument, it must be replaced by 
the mercury piezometer (b), which possesses the advantage that the 
position of the thermometric scale shifts along the stem according 
as the depth varies. Also any number of them can be used at the 
same time at different depths on the same line. In deep ocean 
soundings the combination of this instrument with the water 
piezometer for the determination of both depth and temperature 
independently of the length of the sounding line is invaluable for 
accurate work. The inverting thermometer of Messrs bTegretti and 
Zambra ( c ) is the instrument most suitable for isolated observations. 
It is also of very great use for supplementing and controlling the 
observations with the other instruments, especially in the case of 
sea-lochs or fiords, where the temperature distribution is often much 
disturbed by the imperfect mixture of fresh with salt water. 
For the successful and expeditious carrying out of deep-sea tem- 
perature observations, the investigator should be furnished with im- 
