REPORT ON METEOROLOGY. 
Ml 
it, and an occasional difficulty of arriving at a precise result, we 
may expect the most valuable results from its application to 
hygrometry. 
The dew-point hygrometer of Mr. Jones *, though more sim¬ 
ple and compact, is not so satisfactory in its results; it consists 
of a thermometer with a cylindrical bulb turned upwards and 
half-covered with muslin, which is cooled by pouring ether 
upon it, and the deposition of the dew is observed on the upper 
part of the bulb. A different form has lately been brought 
forward by Mr. John Adie of Edinburgh }', who incloses the 
bulb of a very delicate thermometer in an exterior ball of glass, 
the interval being filled with mercury; and he observes the de¬ 
position of dew on that portion of the outer ball from which the 
covering of muslin for receiving ether has been removed. By 
agitating the instrument at the time of deposition, Mr. Adie 
has been able to get results more closely agreeing with Dal¬ 
ton’s experiment, than by the hygrometers of Daniell and Jones. 
The principle of the instrument is very obvious; from the small 
size of the ball of the thermometer the temperature is more 
accurately found than in Mr. Jones’s apparatus; and the small 
bulk and better conducting power of the medium interposed 
between it and its outer case, render it perhaps more sensible 
than the instrument of Mr. Daniell. To do the latter instru¬ 
ment justice, however, (and from my experience of dew-point 
instruments in their simplest forms, I think the remark of im¬ 
portance,) the temperature at which dew appears should not 
only be noticed, but that at which it disappears. The errors 
of the two must almost always be in opposite directions, and 
the mean should be taken. 
A dew-point hygrometer, in some respects resembling Mr. 
Adie’s, has been proposed in America by Mr. Hayes J. 
Another instrument,—and though I have not tried it, I confess 
it appears to me a very elegant one,—has recently been proposed 
by M. Pouillet§. He places a delicate thermometer vertically 
with its ball upwards, which passes into a small cup of polished 
silver. Ether is poured into the cup till it covers the ball, and 
when by the coolness produced by its evaporation the deposition 
of moisture is produced on the silver, the temperature is noted. 
There is probably no instrument which gives the dew-point 
with so much accuracy as Mr. Dalton’s simple experiment when 
* Philosophical Transactions, 1826. 
f Edinburgh Journal of Science , N.S. i. 60. 
X Sillimans Journal , xvii. 351. § Elemens de Physique, ii. 732, 
Q 
