September 14, 1905 J 



NA TURE 



489 



flour at, say, fifteen to twenty miles per hour, and 

 photographs are automatically taken to show how 

 much dust is raised by the large wheel, which is an 

 ordinary bicycle wheel. The flour is laid only along 

 the centre of the track, so that the bicycle wheel is 

 the only one that causes any disturbance. As the 

 framework is of very simple construction, it may be 

 regarded as causing no serious disturbance in the 

 air, unless it is supplemented by some form of body. 

 In the illustration it is shown provided with a mud- 

 guard and an inclined plane ; these, and any other 

 "bodies," are easily made of cardb6ard, and a 

 number of experiments can be carried out in a com- 

 parativelv short time. 



Bv stripping the frame, the action of the tyre 

 alone, apart from any bodv, may be easily investi- 

 gated in a wav which is impossible with a car having 

 four wheels with a conflicting .set of mud-guards, or 

 a bodv which will affect the back wheels quite 

 differently from the front wheels. Thus Fig. 4 shows 

 the dust thrown up by a tyre pumped hard, from 



Fig. 7. — Dust cloud formed by a square box 3 inches off the ground. 

 Fig. 8. — Dust cloud formed by a square box 9 inches off the ground. 



which it will be seen that the dust is only slight. 

 In Fig. 5 the effect is shown of a very soft tyre, the 

 dust being a good deal more. In both these photo- 

 graphs the tyre was unloaded, except for the pro 

 portion of the frame which it had to bear. But in 

 Fig. 6 the effect is shown of a load of 12 lb. on the 

 tyre, and it is seen that the dust raised is much 

 increased. 



.Mthough, from the point of view of raising dust, 

 tlie tyres may not be so important as the body, their 

 action does seem to be important enough to warrant 

 more careful investigation. Thev are capable, in 

 themselves, of raising a good deal of dust, as is often 

 to be seen in the case of bicycles. Very possiblv 

 dust is carried up by tyres through a kind of suction, 

 and this may vary a good deal with the design. Now 

 that there are so many different tyres, including non- 

 skidding bands, on the market, there is likely to be a 

 considerable difference in the various types as regards 

 dust. In the photographs which are reproduced, the 

 wheel is equivalent to the front wheels of a car, as 

 there is no driving force on the tyre. In the case of 

 NO. 1872, VOL. 72] 



wheels which are drivers, there will be a certain 

 amount of slip of one kind or another, and the dust 

 raised is likely to be greater ; but, actually, there 



nclined pla 



does not seem to be any very great difference between 

 drivers and non-drivers — at least there is much less 

 than would be expected. 



As regards " bodies," Figs. 7 and 8 show the effect 

 of allowing a square box, 18 inches long by 20 inches 

 wide and 12 inches deep, to precede the wheel. In 

 Fig. 7, which shows a large cloud of dust, the bo.x 

 is only j inches off the ground ; but in Fig. 8 it is 

 q inches above the ground. In the latter there is 

 still a lot of disturbance, which fact is of interest, 

 seeing that the bodies of certain cars are brought 

 down to within about 6 inches of the ground. These 

 illustrations and those following are comparable with 

 Fig. 6, the tyre being loaded and soft, and the motion 

 being always from right to left. In Fig. 9 is seen 

 the effect of an inclined plane, so inclined that the 

 air is severely thrown down on to the track. It is 

 a little surprising to notice that the effect of the 

 plane is not nearly so serious as that of the box seen 

 in the two preceding illustrations, although the box 

 and plane are about the same in width, and the 

 |il,ine, which is carried down to within about 4 inches 

 of the ground, is of considerable length. It may be, 

 therefore, that the inclination of the under surface 

 of a car body is not of much importance after all. 

 In Fig. 10 is shown the very marked disturbance 

 caused bv a vertical card, 12 inches square, fixed 



behind the wheel ; this is akin to the tool-box which 

 is often carried low down behind a car, and is 

 generally recognised as having a bad effect. 



