78 The Right Hon. Lord Rayleigh [March 20, 



that I have come acro'^s is that by W. B. Rogers,* who instances 

 their pi'oduction during the bursting of bubl)les of phosphuretted 

 hvdrogen, or the escape of smoke from cannon and from the Jips of 

 expert" tobacconists. For private observation nothing is simpler than 

 Hehnlioltz's method of drawing a partially immersed spoon along 

 the surface, for example, of a cup of tea. Here half a ring only is 

 developed, and the places where it meets the surface are shown as 

 dimples, indicative of diminished pressure. The experiment, made 

 on a larger scale, is now projected upon the screen, the surface of the 

 liquid and its motion being made more evident by powder of lyco- 

 |.)odium or sulphur scattered" over it. In this case the ring is generated 

 by the motion of a half-immersed circular disk, withdrawn after a 

 travel of two or three inches. In a modified experiment the disk is 

 replaced by a circular or semi-circular aperture cut in a larger plate, 

 the level of the water coinciding with the horizontal diameter of the 

 aperture. It may be noticed that while the first forward njotion of 

 the plate occasions a ring behind; the stoppage oi the plate gives rise 

 to a second ring in front. As was observed by Reusch,t the same 

 thing occurs in the more usual method of projecting smoke-rings 1 rum 

 a box ; but in order to see it the box must be transparent. • 



In a lecture given here in IS"??, Reynolds showed that a Helm- 

 holtz ring can push the parent disk before it, so that for a time 

 there appears to be little resistance in its motion. 



For an explanation of the origin of these rings we muse appeal to 

 friction, for in a perfect fluid no rotation can develop. It is easy to 

 recognize that friction against the wall in which the aperture is perfor- 

 ated, or against the face of the cisk in the other form of experiment, 

 will start a rotation which, in a viscous fluid, such as air or water 

 actually is, propagates itself to a finite distance inwards. But although 

 a general explanation is easy, many of the details remain obscure. 



It is apparent that in dealing with a large and interesting class 

 of fluid motions we cannot go far without including fluid friction, or 

 viscosity as it is generally called, in order to distinguish it from the 

 very different sort of friction encountered by solids, unless well lubri- 

 aited. In order to define it, we may consider the simplest case 

 where fluid is included between two parallel walls, at unit distance 

 apart, which move steadily, each in its own plane, with velocities 

 which differ by unity. On the supposition that the fluid also uuO^ es 

 in plane strata, the viscosity is measured by the tangential force per 

 unit of area exercised by each stratum upon its neighbours. AVhen 

 we are concerned with internal motions only, we have to do rather with 

 the so-called " kinematic viscosity," found by dividing the quantity 

 above defined by the density of the fluid. On this system the 

 viscosity of water is much less than that of air. 



* Amer. J. Sci., vol. xxvi. p. 246, 1858. 

 t Pogg. Ann., vol. ex. p. .309, 1860. 



