THERMOMETRY. 
495 
supported on a hook, /. The bulb end may now be gently 
raised or lowered, causing the mercury to flow slowly, 
until the bent part, b, is quite full and the chamber, 
a b } quite empty. At this point the flow of mercury in the 
main tube is arrested, and indicates the exact temperature 
of the bulb or air at the time. On an increase of tempera- 
ture, the mercury will expand, as above explained, into 
the small chamber, a b ; and a return of cold will cause its 
recession from this chamber only until it reaches the dia- 
phragm, b. Any further diminution of heat withdraws the 
mercury from the main tube to whatever degree the cold 
may attain, where it remains until farther withdrawn by 
increased cold or till reset for future observation. By this 
means present temperature may be known, and cold registered 
to any fraction of a degree, observable on the most sensitive 
standard thermometer. 
There is no separation of the mercury in any of its indica- 
tions, and no vicissitudes of climate or transit can disarrange 
the instrument. When out of use, or after a journey, it may 
be that raising the bulb may not at first cause the mercury to 
flow from the small chamber as above ; in such case, however, 
a slight tap with the hand on the opposite end with the bulb 
upwards will readily cause it to do so. 
This invention is well spoken of, and promises to be of 
great value. 
Other self-registering thermometers, both for heat and cold, 
have been invented, but they are not in general use. 
The Solar-Radiation Thermometer is a thermometer with a 
blackened bulb, with a scale divided on the stem, and protected 
by a glass tube acting as a casing. It is mounted horizontally 
on two forks resting on the grass, exposed to the full rays 
of the sun, but sheltered from currents of air. 
The Vacuum Solar-Radiation Thermometer differs from the 
preceding in being entirely enveloped in a glass tube and 
globe, from which the air has been exhausted. The object 
of this instrument is to obtain a measure of the amount 
of the solar radiation freed from the casual effects of 
passing currents of air. Observations in different localities, 
or in the same locality under different circumstances, are thus 
comparable with each other, which is not the case when they 
are made with exposed instruments. 
The Terrestrial Radiation Thermometer resembles the solar 
instrument, except that the bulb is not blackened. 
So much for thermometers as regards their general con- 
structions and different forms. Much more might be said in 
reference to them, but the space at our disposal necessitates a 
limitation. 
