NEW HYGROMETER OR DEW POINT INSTRUMENT. 2] 
instrument, recorded in this paper, and which may be recommended to others. 
The proper mode will be, first, to note the instant of decided appearance of mois- 
ture, and mark the temperature indicated by the thermometer; then, to stop the 
farther reduction the instant this decided deposition is observed; next, to mark 
the instant of disappearance, and the corresponding temperature; and, lastly, to 
take a mean of the two observed temperatures. In observing the point of appear- 
ance, | notice the first apparent indication of moisture, and then see whether, 
immediately on farther reduction of temperature, this indication becomes quite 
distinct. I then take the former indication as marking the temperature of depo- 
sition. In this way we can easily distinguish between a true and a false indica- 
tion. Every one, however, as before observed, must be permitted to make his 
observation in the manner which he thinks best. 
Since this paper was first drawn up, I have made a series of observations, with 
the view of comparing the indications of the instrument with those afforded both 
by DantELL’s hygrometer, and by Dauron’s method of transference, which last is 
generally admitted to be the most trustworthy, although certainly very far from 
being the least troublesome.* These observations have supported the view that 
DanieLi’s hygrometer yields indications somewhat too high, although not to 
quite so great an extent as follows from Mr Aptn’s observations. They shew that 
the tendency of the instrument described in this paper is rather in the opposite 
direction, and that its usual indications are a very litile too low, but that this de- 
viation is less on an average than 1°, and therefore quite within such limits as 
fully to justify reliance on its results. The following table contains the observa- 
tions; those by Datton’s method being stated at the actually observed dew point, 
and the others in excess or deficiency, with reference to DaLTon. 
-| 12.| 13.) 14.| 15.) 16.) 17.| 18.) 19.) 20.| 21. 22, | 23.) 24.) 26.| 28.| 29.| 30. 
33 | 32 | 362) 34 | 42 | 353) 31 | 354) 399] 45 | 43 
+ H+ A+ H+19)—14)+ H+ 4-2 |+12)/+14)— dAverage +0°-29 
—23)/—14)—2 |4+ 3)/-2 |—23)-— 4/-23/— }/— 3/+1}/Average ~0°-95 
Danton, . | 423) 374) 374] 403) 40 | 40 | 35 
DANIELL, . |—1 |+14/+ # = |4+24/+ 3/-1 
New Instru- 
ment, } —1|+2\- H#- 2 —23 
We thus have, in a series of observations of dew point temperatures, varying 
from 32° to 45°, Danrext’s hygrometer, in eighteen observations, giving twelve of 
them in excess or above the dew point by Datron’s method, five of them in de- 
ficiency or below the point referred to, and one of them shewing equality. The 
average of the whole, however, gives only 0°29 F. in excess. I cannot, however, 
help saying, that when I got an observation by Danrzxt in deficiency, and repeated 
* In support of the great trustworthiness of Datron’s method, I may appeal to the opinion of 
Professor James Fores, 2d Report on Meteorology to British Association, 1832; and to Dr Tuom- 
son’s opinion, there referred to. It is unnecessary to say that I had previously compared the ther- 
mometers, and that I made allowance in the comparative table drawn up, for a slight difference ob- 
served in their indications. 
