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rain-drops fall from a sufficient height to attain a terminal velo- 

 city before the close of their descent, and taking as the basis of 

 the calculation the formula of Hutton, which expresses numeri- 

 cally the law of resistance, as determined by experiment, in the 

 case of spherical bodies, Prof Rogers, in the first place, com- 

 puted the terminal velocity of a spherical drop of water one 

 tenth of an inch in diameter. Thence he deduced the velocities 

 corresponding to other successively smaller diameters, by the 

 simple rule that for unequal spheres of like material the terminal 

 velocities are proportional to the square roots of the diameters. 

 In this way was calculated the following table of the terminal 

 or greatest attainable speed of spherical rain-drops, ranging in 

 diameter from one tenth to one four thousandth of an inch. 



These numbers would of course require to be more or less 

 modified, if account were taken of the altered form of the drops 

 as they descend, but as we are ignorant of the nature and 

 amount of this change, we cannot determine its effect on the 

 terminal velocity. 



If instead of assuming the descending globule to consist of 

 water throughout its whole volume, we suppose it to be a hollow 

 shell of water like a microscopic soap-bubble, it is obvious that for 

 the same diameter the terminal velocity would be greatly less 

 than in the above table. Admitting with Saussure and others that 

 clouds are made up of such hollow vesicles of extreme minute- 

 ness, it can be shown that their descent to the earth would be so 

 slow as to make their gravitating tendency inappreciable during 

 the short time in which we watch them as they float by. 



