APPLICATIONS TO MECHANICS 93 



in such a way that the cylinder itself did not appear 

 in the photograph. The cylinder was lifted out and 

 somewhat sharply plunged again into the liquid, and 

 meanwhile a series of photographs was taken, repre- 

 senting the phenomenon at the commencement of the 

 operation. At first there was a progressive series of 



Fig. 60. — Chopping waves of very short period. 



depressions visible along the surface of the water, 

 corresponding to the moment at which the cylinder 

 was lifted up, and then a marked upheaval at the 

 moment the cylinder was again plunged into the 

 water. This upheaval travelled along with diminish- 

 ing amplitude, more or less interrupted by smaller 



Fig. 61.— Advancing wave. 



secondary waves, which advanced with the primary 

 one. The velocity of the wave could be estimated 

 by measuring with a scale the distance travelled by 

 the summit of the wave during the period of one-tenth 

 of a second, which represented the duration of the 

 interval between each successive exposure. Progres- 

 sive waves showed incomplete contours when the 

 chronophotographic method was adopted, and that 

 was because the hinder surface of the wave was the 

 8 



