hours of the day, thiat i itis z 
place cannot be com for any valuable purpose, unless 
the temperature Mente) at the same hours of the day. The 
ere temperature of places can be ascertained only by this 
ity, unless there be a series of observations so exten- 
sive as to lead to the conclusion, that at each place the obser- 
vations must be made at certain ren differing ear 
places, t mean of which will giv 
—. This is'so great i di diffieul a oer it wil 
rarely be a Iti is Poles ‘the less necessary, i 
those thermometers 
vations on grometer, or zero 
forms aa changes of clouds, the fall of reier! snow, dew, &c. 
and all notice of the results already obtained, I propose to 
consider a few points necessary to be attended to, in order to 
make the observations with the thermometer ee in- * 
' teresting and valuable. 
1 curacy of the instrument. The points at which 
mometer may, -however,. be imperfect because the tube con- 
tains a portion of air. If the bore of the tube be. relatively 
large, its Seaton chaise pried ty he soldi al git 
silver passing through it when the. instrument is inverted. 
If the bore be quite small, this effect, howeyer, will not-take 
— even if all the air is excluded, owing to greater friction. 
——— tube. = ——— Ses its diameter.’ In: this 
plage een proved correct only by co the temper- 
ature it shows, with one known pi Reet oon 
‘This source of error is not liable to be great.” j 
. lar. 
_ 2. The elevation of the thermometer above the surface of - 
_the earth. The common elevation of the bulb, is about six 
feet from the ground. Saye ete exposed 
ky, ours Hie of a building 
3. Protection of the th t reflected calorie,” by 
a screen placed nies it, leaving the circulation. of of the air 
entirely free. The heat is often reflected from a road: oz 
large spot of uncovered earth, or from rocks, so. as to raise 
the mercury one or two degrees in a hot day above the tem- 
ofthe air, and that too when the re reflecting surface is 
at the distance of several rods. Such a screen too will lessen. 
the effect of radiation of caloric, by which, under a clear'sky, 
y evident the results at one | 
; 
€ 
