3®4 Meteorological Observations at Hudson^ Ohio. 



move with the same velocity. If we knew their absolute or rel- 

 ative velocity for each observation, it is uncertain what resultant 

 would be obtained ; but as the diurnal change in the dii^ection 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 each other. 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 for 

 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 Prof. 

 Forbes the diminution of temperature to be 1° F. for 352 feet 

 elevation, a complement of the dew point equal to 50° will be 

 required, while the highest observation here has been only 36°. 

 Perhaps the bases of cumulus clouds never attain so great a 

 height ; yet clouds are sometimes seen at a much greater eleva- 

 tion. Thus Gay Lussac, at the height of 23,000 feet, saw clouds 

 above him, apparently at a great elevation ; and clouds are some- 

 times seen above the summit of Chimborazo. All accounts, so 

 far as I have seen, agree in representing the winds on the sum- 

 mits of the loftiest mountains, in all latitudes, as blowing nearly 



