GEOLOGY. 319 



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the minuter details are not shown. On examining the fossil rain marks he 

 had not found that the radiating lines were preserved. They were doubtless 

 destroyed by the drifting in of the new material by which they were covered 

 up. In other respects they resembled recent rain marks, and could be ac- 

 counted form no other way than by the contact of drops of falling water. 



At a subsequent meeting Professor "W. B. Rogers presented the results of cal- 

 culations which he had lately made, of the terminal velocity of rain drops of 

 different diameters. As the impinging force of the drop must in all cases de- 

 pend on its weight and velocity jointly, the determination of the latter quan- 

 tity, even approximately, would seem to be of considerable interest in con- 

 nection with the subject of rain- drop impressions. Were the space around 

 the earth a vacuum, a falling body would continue to be accelerated at a 

 nearly equal rate to the end of its descent, and would not attain its maximum 

 velocity until the moment of its impact on the ground. Such, however, is 

 not the condition of a body descending through the atmosphere. The parti- 

 cles of air lying in its way oppose a resistance to its motion, and this force 

 increases in a very rapid ratio as the velocity augments. There will, there- 

 fore, in every case, be a certain speed at which this resistance, acting upward, 

 will precisely equal the weight of the falling mass, and when this is once at- 

 tained all further acceleration must cease. In these conditions, supposing the 

 air to be of uniform density down to the earth, the body will fall through the 

 remaining distance at a uniform rate. This terminal velocity, therefore, is ob- 

 viously the greatest speed which, under the conditions, the body can acquire 

 by descending through the air, however great the altitude from which it may 

 be supposed to fall. 



Assuming, what is probably in most cases the fact, that the rain-drops fall 

 from a sufficient height to attain a terminal velocity before the close of their 

 descent, and taking as the basis of the calculation the formula of Hutton, 

 which expresses numerically the law of resistance as determined by experi- 

 ment in the case of spherical bodies, Professor Rogers, hi the first place, com- 

 puted the terminal velocity of a 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 attain- 

 able speed of spherical rain-drops ranging hi diameter from one tenth to one- 

 fourthousandth of an inch. 



These numbers would of course require to be more or less modified, if 



