-1859] WRITINGS OF JOSEPH HENRY. 247 



mathematically) must not exceed the seven-thousandth of 

 an inch in diameter. Now the particles of a cloud are 

 sometimes known to present the appearance of similar colors, 

 and therefore are not larger than those of the lycopodium. 

 This extreme minuteness is sufficient to account for the sus- 

 pension of clouds or the extreme slowness with which they 

 descend. M. Maille of Paris has attempted to compare the 

 volume of a particle of this size with that of a drop of rain 

 water of about a tenth of an inch in diameter. He finds that 

 it would require upwards of 200 millions of particles of cloud 

 to make one drop of rain water of the size mentioned. We 

 are prepared to admit the correctness of the conclusion when 

 we reflect on the rapid increase of the volume of a sphere 

 relative to the increase of its diameter. For example, if a 

 series of spheres have diameters in the ratio of 1, 2, 3, 4, 5, 6, 

 the volumes or weights of the spheres, provided the}^ are of 

 homogeneous material, will be represented by the numbers 

 1, 8, 27, 64, 125, 216. Indeed nothing is more deceptive than 

 the estimate we form of the relative volume or weight of 

 different solids by simply comparing their diameters. It 

 requires but a very small increase in the diameter of an egg, 

 for example, to double its weight. We know that the re- 

 sistance of the air to the descent of a falling body is in 

 proportion to the surface which it presents to the resisting 

 medium. Now every time a drop of water is divided, a new 

 surface is exhibited, and when the division is carried as far 

 as that of the particles of cloud, the resistance must be so 

 great that an indefinite length of time must be required to 

 produce a descent of a few hundred feet. 



The process of the formation of clouds will be described in 

 a subsequent section ; we may here however mention that 

 the forms and aspects in which they are presented are indic- 

 ative of the circumstances in which they are forming or dis- 

 sipating, and hence the importance of giving special names 

 to these forms in order that they may become objects of defi- 

 nite study. The first attempt at a descriptive classification 

 of clouds was by Mr. Luke Howard in 1802. An account of 

 this is given in all works on meteorology, and we need here 



