324 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
F. C. R. 
85 
80 
75 
70 
65 
60 
55 
50 
45 
40 
35 
30 
Z5 
Cut 46. — Comparative ther- 
mometric scale Falireiiheit, 
Celsius, and Keaumur. 
lo set the instruineiit for oliservation it is only necessary to 
j)lace it bulb downward, when the mercury takes the temperature, 
just as in an ordinary thermometer. If at any time or place the 
temperature is required, all that has to bo done is to turn the ther- 
mometer bulb upw.ard and keep it in this position until the reading 
is taken. This may be done at any time afterward, for tlie quantity 
ot mercury in the lewder part of the tube which gives the reading is 
too small to be sensibly aflected by a change of temperature unless 
it is very great, while that in the bulb will continue to contract 
with greater cold and to expand with great lieat. In the latter case 
some mercury will pass the contraction a and may fall down and 
lodge at 1>, but it can not go further so long as the bulb is upward, 
and thus the temperature to be read -will not be affected. 
Now, whenever the thermometer can bo handled it can readily 
be turned bulb upward for reading the existing temperature. It 
must bo clearly understood that this thermometer is only intended 
to give the temperature at the time and ^dace where it is turned 
over; it is simply a recording thermometer. In its present state it 
can not be used as a self-registering maximum and minimum, though, 
if re(£uired, it could be constructed to act as a maxinmm. 
In order to make the thermometer jierfectly satisfactory, it was 
necessary to protect it from pressure as well in shallow as in the 
deepest seas, for in either case the pressure would cause au error of 
greater or less degree in its indications. Like an ordinary ther- 
mometer it is devoid of air, and so quite different from Sixe’s, which, 
containing compressed air, h.as a certain internal resistance. Hence 
it would be more affected by pressure than Sixe’s thermometer, 
however thick the glass of the bulb. By the simple expedient of 
inclosing the thermometer in a glass shield, e [plate xxi, fig. 2], 
hermetically sealed, the effect of external pressure is entirely elimi- 
nated. The shield must of course be strong, but not exhausted of 
air. It will, however, render the inclosed thermometer less readily 
affected by changes of temperature, making it more sluggish. 
To counteract this tendency mercury is introduced into that 
portion of the shield surrounding the bulb, and confined there by 
a partition, d, cemented in the shield around the neck of the ther- 
mometer bulb. This mercury acts as a carrier of heat between the 
exterior of the shield and the interior of the thermometer; and the 
efficacy of this arrangement having been experimentally determined, 
the instrument has been found far superior in sensibility to Sixe’s. 
So long as the shield withstands the pressure — that is, does not 
break — the thermometer will be unaffected by pressure, and there 
is abundant experience to show that such a shield will stand the 
jirossure of the deepest ocean. Doubtless the shield wdll bo slightly 
com])ressed under great pressure, but this can never cause an internal 
jiressure sufficient to have an appreciable effect upon the thermome- 
ter. This method of shielding is, therefore, quite efficacious, and 
deep-sea thermometers so protected do not require to be tested for 
pressure in the hydraulic press. They simply require accurate tests 
for sensitiveness and for errors of graduation, because they are 
standard instruments adapted to the determination of very small as 
well as great differences in temperature, some one or two tenths of a 
degree in shallow water. The test for sensitiveness should determine 
the time the instrument re(iuires to take ui> a change of 5'^, rise or 
fall, and the time is found to be from five to ten seconds. 
Thus, provided the turning-over gear is found to answer, this 
instrument evidently possesses great advantages. It has no at- 
tached scale, the figures and graduations being distinctly marked 
on the stem itself, and the shield effectually preserves them from 
