352 On a Portable Hygrometer. 
cessary to nearly fill it when the bulb occupies its axis ; this quanti- 
ty once adjusted, serves for many subsequent experiments; the tube 
is then screwed to the cap, and the instrument being held in an in- 
clined position, the thread on the inferior part of the tube is wetted 
with ether ; in a few moments dew is deposited on the platina ferule,— 
the instrument is then agitated, to ensure an uniform distribution of 
heat within the tube ; the dew begins to disappear,—the temperature 
then indicated by the thermometer is carefully noted. In the section- 
al representation of the instrument, A is the brass tube with its cotton 
envelope, B C the platina or steel ferule, D disk of caoutchouc and 
screw cap, E the thermometer. 
_ Another form of the instrument, more elegant and costly, and fitted 
only for determining the temperature of vapor of low tension, has 
been used. S No. 2, represents a black glass tube, of the same capaci- 
ty compared with the bulb as the brass cube; it is attached by fusion to 
the stem at R, and after being filled in part with alcohol, is hermeti- 
cally sealed at O. The upper part of the black tube is the deposit- 
ing ‘surface, the lower heing covered with thread; the same mode of 
experimenting as in the former case. 
For making corrections in barometrical 
experiments, this instrument is peculiar- 
ly fitted, the “ Detached Thermometer” 
serving to determine the temperature of 
the air and tension of atmospheric vapor ; 
the experimental result is substituted fory 
that deduced by calculation from the gen- 
eral formula of LaPlace. Some results 
recently obtained by my friend, Dr. Ed. 
E. Phelps, show the great importance of 
making the experimental, rather than the 
caleulated correction. 1B 
~The small size of the instrument, ena- 
bles us to use a thermometer with large 
divisions, the dew point can be found to a fraction of a degree; the 
quantity of matter to be refrigerated being small, a reduction of tem- 
perature is speedily effected, and only an inconsiderable quantity 
cesoen is required. 
a erg Noy. 1829. 
