56 THE POPULAR SCIENCE MONTHLY. 



of using the legs should be adopted by man as is displayed in the 

 model swimming of that amphibian. 



In analyzing the stroke of the frog, we notice that there is no ver- 

 tical motion ; the whole direction of the force is in a plane exactly 

 horizontal, and is accomplished by virtually opening and closing the 

 space between the knees offering the sole of the foot as a resistance 

 while kicking, and placing the feet in a position of least resistance 

 while recovering. 



In accomplishing the first of these conditions the opening and 

 closing of the space between the knees the knees should be thrown 

 out, and the contraction of the legs made slowly, in order to cause as 

 little resistance as possible to the headway already attained. 



It will be found that, if we alternate the stroke of the arms and 

 legs by giving propulsion with one while recovering with the other, 

 a more constant buoyancy will be attained, and, for long swims, it will 

 be found far less fatiguing. 



-- 



HOW THE ANCIENT FOKESTS BECAME COAL* 



By M. G. De SAPORTA. 



THE carboniferous formation represents the most wonderful epi- 

 sode in the history of our globe. It gives us an impression com- 

 parable in strangeness to that produced by those wonderful civiliza- 

 tions which blossomed out so suddenly and so splendidly in the in- 

 fancy of mankind. Only a rare concurrence of circumstances could 

 have brought on the expansion of plant-growth which characterized its 

 epoch. The world of plants was still young and imperfect. Vegeta- 

 tion was characterized by the abundance of green parts susceptible of 

 rapid growth, and of an almost indefinite development. It was, how- 

 ever, destitute of two characters which have been acquired by the later 

 plants : those of the periodical and gradual increase of parts destined 

 to endure, and of an absolute specialization of the reproductive appa- 

 ratus. The vegetable kingdom was the first factor in the pi'oduction 

 of coal, but not the only one ; and two other factors must be taken 

 into account in studying its genesis. One of these related to the con- 

 ditions of the environment, the climate, and the temperature ; the 

 other to the situations in which the plants that were converted into 

 coal were placed. Had either of these conditions been essentially 

 different or left out, we would have had no coal. The influence of 

 situation is shown by the fact that the coal-beds are always intermit- 

 tent ; that they are limited in extent, and pass laterally into shales 



Translated and condensed for " The Popular Science Monthly" from the "Revue 

 des Deux Mondes." 



