324 Meteorological Observations at Hudson, Ohio. 
move with the same velocity. If we knew their absolute or tel- 
ative velocity for each observation, it is uncertain what resultant 
would be obtained; but as the diurnal change in the direction of 
the clouds appears almost or quite insensible, it may be presumed 
that the diurnal change in their velocity must be small; so that 
it appears probable that the mean of the above observations, 
S. 88° 13’ W. represents very nearly the absolute progress of the 
atmosphere in the region of the clouds. 
There is a very simple and obvious corollary arising from the 
constancy of the mean pressure of the atmosphere, which I do 
not recollect ever to have seen noticed, but which deserves to 
be kept constantly-in view, viz., that over any parallel of lati- 
tude the mean northerly and southerly motions of the entire 
atmosphere must be equal to eachother. One half of the atmos- 
phere is below the height of 3.44 miles. According to obser- 
vations given in Dalton’s Meteorological Essays, about two fifths 
of all the clouds observed in England.in five years were above 
1050 yards high. The mean height of the clouds is probably 
greater in the United States, yet there would seem no room 
doubt that the clouds here. generally float in the lower half of 
the atmosphere. The observations then on the preceding page 
exhibit mainly the progress of the lower half of the atmosphere. 
It would be desirable to separate the two classes of observations, 
yet it is difficult to accomplish this satisfactorily. Mr. Espy has 
proposed to determine the height of the bases of cumulus clouds 
by the rule, that this height in feet is equal to the complement 
of the dew point multiplied by 300, “In order that the base of 
the-cloud may reach to the height of 3.44 miles, the comple- 
ment of the dew point must be 60°, which is much higher than 
has ever been observed in Hudson. If we assume with F 
Forbes the diminution of temperature to be 1° F’. for 352 feet 
elevation, a complement of the dew point equal to 50° will be 
