492 
POPULAR SCIENCE REVIEW. 
It is not known wken and by whom the thermometer 
was first invented; but in 1590, Sanctorius, of Padua, con- 
structed an air-thermometer. About the middle of the 
seventeenth century a great improvement was effected by 
Italian artists, who, under the direction of the members of the 
Academia del Cimento, made spirit-thermometers, as before 
mentioned. 
The idea of using mercury is said to be due to Halley ; but, 
according to Boerhaave, it was Olaiis Bomer — known to 
astronomers as the inventor of the transit instrument, and the 
discoverer of the motion of light — who first constructed a 
mercurial thermometer, in 1709. It was, however, Fahrenheit, 
of Amsterdam, who perfected Bonier’ s invention, by devising 
the thermometer which now bears his name. M. Breguet, of 
Paris, has constructed a thermometer founded upon the unecpial 
dilatation of different metals. It is, however, more curious 
than useful. 
The differential thermometer consists of a horizontal glass 
tube, furnished with bulbs at each end, of the same size, bent 
twice upwards at right angles. In the horizontal part of the 
tube a small quantity of coloured liquid is placed. Atmospheric 
air is contained in the bulbs and tube, but separated into two 
parts by the column of liquid. The instrument is so adjusted 
that when the drop of liquid is at the middle of the tube the air 
in the bulbs has the same pressure ; and having equal volumes, 
the quantities on each side of the liquid are necessarily equal. 
If the bulbs be affected by different temperatures, the liquid 
will be forced towards that side on which the temperature is 
least, and the amount of the departure from the zero or middle 
of the scale may be ascertained by inspection. The sensitive- 
ness of this instrument is very great, and it is this feature 
which renders it of great value in delicate physical observations. 
It being desirable that the liquid should be one which does not 
give off* vapour at ordinary temperatures, diluted sulphuric acid 
coloured with litmus is usually employed. The differential 
thermometer was invented by Sturmius, of Altdorf, towards 
the close of the seventeenth century. Its use was revived by 
Sir J ohn Leslie and Count Bumford, in 1 804. Leslie's con- 
struction varies from Bumford's (to which the preceding de- 
scription specially applies), in having the vertical arms longer 
than the horizontal one, and in having two scales, one attached 
to each vertical arm. 
It is of great importance in meteorology that the observer 
should be able to ascertain the highest or lowest point of the 
thermometric scale, which the column of mercury may have 
reached during his absence, and several contrivances are in 
use to obtain a record of this kind. 
