Meteorological Observations at Hudson, Ohio. 277 
1843, July 1, 831° 
ne TG, 80:0 
5 aie, 81:3 
The last of these cases was immediately followed by a thunder 
shower. 
The following are the only instances in which the dew point 
has sunk to zero. It should be borne in mind that these obser- 
vations are made only at 9 a.m. and 3r.m. If they had been 
made at sunrise, the list would have been larger. 
1843, February 16, 9 a. M. —3'5° 
ii4 LT, 6e : ed 6:0 
e se 3 P.M. —12 
March 23, 9 a. M. —0:3 
Some individuals whose opinions are entitled to great respect, 
have expressed dissatisfaction with Daniell’s hygrometer. The 
following objections have been urged against it. 
1. That the instrument is not susceptible of sufficient accuracy. 
2. That the observation requires considerable time. 
3. That it is not always practicable. See this Journal, Vol. 
XLVII, p. 19. 
The first objection has weight in the hands of an unskillful 
person. When an excess of ether is used, and sudden cold is 
produced, the thermometer does not instantly indicate the cold 
generated, and if you note the thermometer at the instant the 
ring begins to form, the observation is too high. If there isa 
copious deposition of dew, it requires some time for it to be re- 
dissolved, and the thermometer meanwhile will rise above the 
dew point, so that both observations will be too high, and the 
result may be erroneous several degrees. But if we use barely 
sufficient ether to produce a visible ring of dew, one observation 
will be about as much below the truth as the other is above, and 
the mean of the two may be relied upon within a degree. 
At one time I found my assistant was habitually getting a very 
high dew point, which excited my suspicions, and on examina- 
tion I found that the ether had all distilled over into the upper 
ball. The bulb of the enclosed thermometer was of course left 
entirely insulated. The result was that he got no ring of dew, 
and but little depression of the thermometer. 'The precaution 
should always be observed to expel the ether into the lower ball 
Vol. xx1x; No. 2.—July-Sept. 1845. 36 
