Sd + 
334 Prof. W. A. Norton on the Diurnal and Annual 
“mented quantity of vapor, and the barometer falls until 3 or 4 
_p.m. When the temperature begins to fall the barometer also 
~ descends by reasan of the increase in the density of the air: bs 
in the evening, when a portion of the atmospheric vapor begin 
to fall in dew, -a tendency to a diminution of the sal 
pressure‘arises,. but it is not until-about midnight that this ten- 
dency begins to prevail over the tendency to an increase. From 4 
that time the barometer son from this cause, until towards the | 
hour of minimum temperatu | 
KS The explanation which i hee before given of the diurnal 
Ea ‘variations of the Pode force is, in wera en as follows. In 
the morning as the temperature rises the molecular magnetic 
force increases,’and therefore the hor condita force tends to in- 
crease, but the diminution arising from the evaporation going on 
~at the earth’s surface over-compensates this tendency, and hence 
the-horizontal force, on the whole, decreases. This continues 
‘until about 10 4.°%. After this the increase, from the risé of 
ee Somat prevails: over the diminution produced by the con- 
ee nued:evaporation* until about 4 p.m. From that time the’ hori- 
5 tones force decreases with ss temperature anil about 11 P.M. ; 
when?the tendency to ‘increase resulting from the deposition ' of: 
dew begins to prevail over the opposite tendency resulting from 
the falling of the temperature. ‘This secon pecentation CORE 
tinues until about 5.4. M. 
In view of what»has now been stated it ‘will, eS think, be be ad- 
mitted that the diutt al,variations bath of the hérizontal magnetic 
fi of. ‘baréiieter are in all: ‘probability: certain . —_ 
resulting from the joint-operation, of the same two geneval.antag- 
onistic causes—viz., variations of temperature. is ‘variations of ~ 
humidity. ‘The theory of the variations of the horizontal force 
which I have advanced js ‘n'a accordance with: the faét of the ap-- 
- Parent connection subsisting between these phenomena. Other - 
theories may perhaps be devised, equally in accordangg with. this 
singular fact ; and indeed it must be conceded. that. it is eed 
sr of the idea that the cause of the variations of the 
izontal force, like that of the variations of the. barometric 
en must achane in the atmosphere. In fact if it be: admit- 
ted that the particles of the atmosphere and of the atmospheric 
vapor have a magnetic action upon the needle, like those of the 
solid mass of the earth, then it is a simple consequence of the 
ee of the general theory under consideration that the hori- 
ontal force will vary, by reason of this papi after the manner 
in which it is observed to do. For, the greater portion of the 
amigas being posited above the saan: “the tendency of its 
Rn ee 
_ * It is to be observed that we are here —— of the average state of things in 
the course of a year, or quarters of a year. a 
aa we F 
