85 
of Edinburgh, Session 1878-79. 
diminish as the pressure increased, and if the temperature in- 
creased with the depth, the apparent volume of the liquid would 
diminish at a slower rate ; but it would be always possible to deter- 
mine the true temperature as long as it did not increase at so great 
a rate as to dilate the liquid more than it was compressed by the 
increasing pressure. For the investigation of seas such as the 
Mediterranean this form of instrument is most valuable. The 
method of determining accurately both depth and temperature 
from the combined readings of a mercury and a water piezometer is 
explained in the paper communicated to the Chemical Society and 
above referred to. 
In the great majority of cases the most convenient instrument 
to use is the form of Six’s thermometer with protected bulb known 
as the Millar-Casella thermometer, with the following additions 
and improvements, which Mr Casella has applied to them at my 
suggestion : — The size of the instrument is increased so that the 
degrees are wider apart, a degree Fahrenheit on the minimum 
leg occupying about three millimetres of its length. Besides the 
scale of degrees which is attached on enamelled slips to the vul- 
canite at the sides of the stem, there is an arbitrary (millimetre) 
scale etched on the stem itself. The values of the divisions 
of this scale are ascertained by a careful comparison with a 
standard thermometer. It is thus possible to read with cer- 
tainty to a quarter of a millimetre or a twelfth of a degree Fahren- 
heit. The errors due to the scale not being rigidly attached to 
the thermometer, and to the difficulty of determining the height of 
the index by reference to a scale at the side of instead of over 
it, are eliminated. Finally, by having the ordinary scale at the 
sides, the instrument can be used independently of the arbitrary 
scale, and, even where the arbitrary scale is principally relied on, 
the scale of degrees enables the observer to know very approxi- 
mately the true temperature at the moment of observation without 
reference to tables, and, by noting on every occasion the reading 
on both scales, the chance of errors from misreading is greatly reduced. 
The maximum leg, which is only rarely used, is of larger bore 
than the minimum one ; the degrees, therefore, are closer, and the 
temperature of the instrument may rise as high as 100° F. without 
the index entering the terminal bulb. This is a detail of eonsider- 
vol. x. 
M 
