169 
rapidly enough ; so to further test their action I several 
times removed one from my room to my box out of doors, 
and vice versa , and then compared, to see how soon the 
instrument removed corresponded with the one which had 
not been recently changed, and I found that it was several 
hours before the readings were similar, and further noticed 
that when brought from the cold air to the warm room the 
readings were much sooner reliable than when it was 
removed to the cold. 
As an example similar to several others, hygrometer No. 
1 had, on January 7th, been out of doors for some weeks, 
when I put No. 2 out from my room, at 10 a.m., when it 
stood at 32*5. No. 1 stood at 66 out of doors. The course 
of the two, neglecting small fractions, is shown in the fol- 
lowing tables 
Jan. 7th. 10 a.m. 
11 a.m. 
1 p.m. 
3 p.m. 8th. 
9 a.m. 
11 a.m 
1 p.m. 
8 p.m. 
No. 1. 66 
61 
50 
52 
73 
62 
51 
42 
No. 2. — 
47 
56 
59 
72 
61 
51-5 
42*5 
Difference . . 
.-14 
+ 6 
+ 7 
-1 
-1 
+ 0-5 
•f 0*5 
Jan. 9th. 
9 a.m. 
11 am. 
1 p.m. 
3 p.m. 
Jan. 10th, 9 a. 
m. 
No. 1. 
55 
52 
42 
38 
82 
No. 2. 
54 
52 
41*5 
38 
80 
-1 
0 
-0-5 
0 
-2 
From which we 
see that in the 
first 
hour 
it had only 
risen 15 per cent, that afterwards it became too high, and 
was not reliable all that day. It was, of course, a severe 
test for the instrument, but as we have had as great a dif- 
ference as 40 per cent out of doors in one day, any instru- 
ment which changes so slowly is unsuitable for exact meteor- 
ological work. 
Besides this slowness of action these two hygrometers, 
which I have reason to believe are first-rate instruments of 
their kind, will not always give similar indications* For 
instance* when 
