Correspondence.

Progress, Volume IV, Issue 9, 1 July 1909, Page 307

Correspondence.

(To the Editor Progress.) Eoa, 12th May, 1909. Sir, — In leieienee to Mr. Poolman's queiy about tb,e speeds of diffeient portions of the chcumference o£ a wheel in motion on a road, I have a word to say. If your correspondent will take any wheel running on lails or on the ground, and maik with chalk the top and bottom of the wheel, the rail immediately under the marked bottom of th|e wheel, and the frame of the vehicle over the marked top of the wheel in such a manner that the four marks are in the same plane, then move the vehicle forward a few inches h,e wall find that no measuiement will be necessary to demonstrate the fact that the top of the wheel has travelled further than the bottom in the same time; consequently must have tiavelled at a higher speed. The same result will be shown if a photogiaph is taking of a moving cartwheel — • the bottom of the wheel will come out sharp and distinct, but the top will be blurred, proving that the lelative speeds of the top and bottom could not be the same. The difference between the lelative speeds of the top and bottom will depend on the diameter of the wheel. If your correspondent, after trying the experiment mentioned above, will return the wheel to its starting point and then raise it slightly above and clear of the rail and repeat the experiment by turning the wheel on its axle, he will find the lesult very different — the speeds of the top and bottom will now be the same. The difference in the results is due simply to the fact that in the first case th© axle moves forward as the wheel revolves, and in the second case it lomains in the same position. — I am, etc., Sigma.

(To the Editor Prouress.) Dunedin, May 17th ; 1909. Sir, — Noticing a letter in Progress about whether a wheel moving along the road moves taster on the top than on the bottom, I forward enclosed a photogiaph which will, I think, explain e\eiytlnng. — I am, etc., Floub Miller. [We ha\e duly received a photogiaph, a cutting irom the "Strand" Magazine, which is thus described in the accompanying letterpiess. ' ' Revealed by the Camera. This curious photo, taken with; an exposure too slow for the movement of the motor-car, illustrates the wellknown though startling truth that the top part of a wheel running on the road moves much faster than the lowei pait of the same. The back wheel is seen quite blurred round the top, while the lower pait is distinct and apparently still." The Editor of the Strand vouches for the genuineness of the picture. The demonstiation seems to be complete, and would be so but ±01 the fact that the blur in the photo may be accounted foi just as well by the different incidence of the light on the tops and bottoms of the wheels, as by the different speeds of the top and bottom. We should have been glad to reproduce the pictuie, but we feel sure it is not good enough for reproduction. We have no hesitation in saying that the picture certainly beais out the explanation of the Strand. — Ed.P.]