112 Mr. W. Sutherland on 



of the motion of natural liquids both viscosity and capacity 

 for exerting tension have to be taken into account. Of 

 course one gathers either directly or indirectly that most of 

 the great writers on hydrodynamics, beginning with, say, 

 Poisson and Stokes, saw the philosophical necessity for 

 recognizing tension in liquids in motion ; and Maxwell, when 

 introducing his idea of the " time of relaxation " of an impul- 

 sively generated tensile strain, intended it to apply even to 

 natural gases as well as to liquids. The merit of the " ducks 

 and drakes " phenomenon is that it brings liquid tension from 

 the region of scientific imagination to that of actual fact, and 

 demonstrates in a brilliant manner that tension in moving 

 liquids is no mere subordinate matter only slightly altering 

 the properties of the liquid from those of the ideal perfect 

 one, but produces a fundamental change. Of course static 

 tension, of which Worthington appears to be the latest inves- 

 tigator, has already received a certain amount of practical 

 attention. 



The broad facts of il ducks and drakes 3i are, that a solid 

 body having a flat face and made to impinge with this face 

 on a liquid surface parallel to it tends to rebound from the 

 surface if the component velocity parallel to the surface 

 exceeds a certain value ; the tendency depending on the rela- 

 tion between the area of the face and the mass of the body, 

 and also on the angle the direction of its velocity makes with 

 the normal to the liquid surface : the larger the mass of the 

 body per unit area of flat face the smaller is the ratio of the 

 normal velocity before impact to that after ; the smaller the 

 angle that the velocity makes with the normal the smaller is 

 the ratio of the normal velocity before impact to that after, 

 until a limiting angle is reached at which the ratio appears 

 to be zero, so that for smaller angles the rebound ceases 

 entirely. Of the existence of this limiting angle with ordi- 

 nary stone, glass, and oyster-shells, and sea or river water, I 

 have satisfied myself by many trials, and I have thought of 

 making a quantitative study of the laws of the rebound of 

 solid from liquid, but see no immediate prospect of the neces- 

 sary time. But for present purposes the existence of the 

 limiting angle is an important fact, because it implies also 

 what has been already stated, namely, that with a given 

 normal velocity there must be associated a velocity parallel 

 to the surface not less than a certain value if there is to be 

 rebound ; and the increasing efficiency of the rebound with 

 increasing angle implies that the velocity parallel to the 

 surface is the most important element in the conditions of 

 rebound. Evidently there is an entire difference between 



