336 Prof. W. A. Norton on the Diurnal and Annual 
tween the maxima of the one and the minima of the other.* In 
the midst ofsirregular variations the following general law is very 
another maximum about August, and another less decided mini- 
mum in the fall. This law has also been revealed by the 
servations made in Europe. It is stated as follows by Raatoes, 
(Cours. Complet de Meteorologie, p. 282,) “Setting out from 
winter the pressure” (of the air) ‘ediminishes until the equinox, 
then it augments:in summer without attaining nevertheless to the 
winter mean; we afterwards find in autumn traces of the second 
minimum, then the curvétises again until winter.’? The same law 
is‘also manifest in the curve for the horizontal force, substituting 
‘maximum for minimum and minimum for maximum. Before 
“noticing this connection between the annual variations of the hori- 
- zontal force and of the height of the barometer, I inferred from 
my previous investigations that the following was probably the 
eye papiapation of the annual seers of the horizontal force. | 
rom the winter.to the summer this force tends to increase by 
reason of the increase of pesitoahewire and to diminish by reason of 
he loss of moisture from the earth’s surface (or increase of vapor 
in the air). Fora time the first cause prevails over the second and xs 
_ thus the force augments. But the formation of vapor will be pro- ~ 
ra thus it happens that some time during the spring the ten- 
toa beahsna, of. the horizontal force tomes | to prevail 
pee r the ten increase This state. ‘of ‘things | contin- 
tae Giasil thet aedEMioacicnom ‘temperature’ (July or August), 
After ~ for a certain’ iain the temp apres ‘will fall without a 
nate diminution in the quantity of vapor in the air ; and 
thus a horizontal force will contin 4 to decrease for a certain 
ts 
time beyond the time of maximum temperature. But we may 
which will continue until the effect of the average daily 
r (being proportionally less from day to day) comes to be 
_ overbalanced by the opposite effect of the diminution of temper- 
ature. Thus there may be a second maximum in the fall, follow- 
ed by a minimum in the winter. While this would be the gen 
eral course of things, - would be room for material irregular- 
ities in individual yea 
is explanation of the annual variatiens of the horizontal 
magnetic intensity I find to be substantially the same, mutatis 
mutandis, as that given by Professor Dove of the corresponding 
variations of the height of the barometer. He has established 
* See Plates in Report of Observations at Girard College Observatory: 
