July 24, 1879] 



NATURE 



29s 



speed, which decreased slightly during the experiment, 

 and would have caused an increase in the coefficient of 

 friction had it not been counteracted by the element of 

 time. 



There is nothing unnatural in the fact that friction de- 

 creases with speed. Friction is mechanical work ; it re- 



li- . 





I'lagnans^Kr/in^ G>^iicii!nt cffriaixjn as affrcted 'byHnie. 



quires a definite force to move a body which is in contact 

 with another, and such movement causes a perceptible 

 wear of the surfaces in contact. The manner in which 

 this work is accomplished can be explained only by the 

 fact that the surfaces in contact are not perfectly smooth, 

 but irregular, although this irregularity may not be dis- 

 tinctly visible to the naked eye. These surfaces, if exa- 



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X— X — X—- X— *( 



— %.—*. — It. — x-^x — X— • 



°~r'^^'^'i'!'l'l'r'i'|M'l'l'l'l' IM 'l' l ' | i|i| M ' l 'l ' l'l't'l' P l'l 

 Seconds o 1 13 4 5 6 f 8sionni3t4t34er7i3is2PsfS2:fvsaitm8 2»zf 



mined under a sufliciently strong microscope, would be 

 found to be somewhat as represented in the accompanying 

 diagram. No. 9. 



If the upper body be moved in the direction of the 

 arrow, s, by a force, P, the point, a, of the upper sun'ace 

 would mount the incline, formed by the corresponding 



portion of the lower body, until it reached its summit 

 at a' ; from this moment it would begin to descend 

 the next incline, from a' to b; provided the force, p, 

 acting in the direction of the arrow, S, would leave it 

 time to do so, the incline from d to c would have to be 

 mounted next, causing a certain amount of resistance 

 during the time the body traversed the distance dc. But 

 if we increase the speed in the direction of the dart S, so 

 that the body will require less time to traverse aVthan to 

 fall through dd, in such case a' would not arrive at c", but 

 at some other point, 6', and then only the portion of the 



incline i' c would have to be mounted, presenting a smaller 

 amount of resistance than in the former case. This 

 illustrates what occurs.^ 



The fact that the coefficient of friction diminishes with 

 speed sufficiently explains why a skidded wheel affords 

 less resistance than one which still rotates, because the 

 resistance occasioned by the rotating wheel is only limited 

 by the adhesion of the wheel on the rail, and this, as 

 already shown, is the same as static friction, since the 

 point of the wheel is stationary as regards the forward 

 movement of the train at the moment it touches the rail ; 

 whilst when the wheel is skidded and slides, the friction 

 is that due to the speed at which the wheel moves on the 

 rail, and is therefore less than the other. 



Douglas Galton 



{To he continued.^ 



GEOGRAPHICAL NOTES 



On Monday the French Geographical Society held an 

 extraordinary meeting, in the large hall of the Sorbonne, 

 for the reception of Major Serpa Pinto, the African ex- 

 plorer. We understand that Major Serpa Pinto and Lieut. 

 Lucien N. B. Wyse have both promised to attend the 

 coming meeting of the British Association at Sheffield, 

 and will give accounts of their recent explorations in Africa 

 and the American Isthmus. Some ather very interesting 

 papers, we hear, are in preparation for the geographical 

 section, over which Mr. Clements R. Markham will pre- 

 side. 



The principal novelty in this month's Petermann's 

 Mittheilungen is an elaborate paper " On the Geographical 

 Distribution of some Plague Epidemics," by Dr. Carl 

 Martin. Dr. Emin Bey has a short paper " On the 

 River Obstructions of the Bahr el Jebel," and we regret 

 to say, Dr. Gerhard Rohlfs writes from Bengazi on June 

 lo, that he has resigned the leadership of the expedition 

 of the German African Society, which was organised for 

 the purpose of reaching the Congo by starting from'Tripoli- 

 Dr. Rohlfs gives as his chief reason for resigning, the 

 length of time the expedition is Ukely to last, and the 

 value of even a single year at his age. He has, however, 

 done his best to remove all difficulties from the way of the 

 expedition in setting out, and these have not been few. 

 He proposes Dr. Stecker to succeed him, and hopes the 



■ This simple illustration is Uken frum an article in the Chicago Railaay 

 CazetU, by M. Krajewski. 



