June i i, 1908] 



NA TURE 



129 



new movement, young engineers found themselves 

 required to learn for the purpose of examinations 

 " facts " which they were well aware from their own 

 experience to be misrepresentations of the real state 

 of affairs. Among the men who have since helped to 

 put these students into the right path and to help them 

 on the way, none were more prominent or are more 

 deserving of praise than Prof. Perry. 



In his laboratories the phenomenon of friction, to 

 take this same instance, is dealt with in such a way 

 as to give the student opportunity to e.xercise all 

 his knowledge of mechanics. Thus he has to test 

 the effects of friction in every part of a mechanism — 

 he is not allowed to forget its existence or to have 

 his mind taken away from it, as may happen so easilv 

 when working among the large and complicated 

 machines of the engineering laboratory. Even in so 

 simple a case as that of the spinning of a horizontal 

 fly-wheel by means of the unwinding from its axis of 

 a rope which passes over a pulley, and carries a weight 

 at its far end, the number of problems that arise is 

 very numerous. To instance the variety of inform- 

 ation which can be derived even from such a simple 

 experiment as this, we may quote the following from 

 Prof. Perry's " England's Neglect of Science " : — 



" Let us take this well-used fly-wheel. The M of 

 a fly-wheel, multiplied by the square of its number 

 of revolutions per minute, gives the kinetic energv 

 stored up in it in foot-pounds. You are asked to 

 measure experimentally the M of this fly-wheel; the 

 loop at the end of a cord goes over the pin .\ on the 

 spindle, and is wrapped n times round the spindle, 

 then goes over the pulley B, and has a weight W at 

 its end. At time O the wheel is let go; in ?, 

 minutes — carefully observed — the cord drops off; in 

 t, minutes from starting the wheel has been brought 

 to rest again by friction. The weight W lb. multi- 

 plied by the height in feet through which it has fallen 

 gives the energy stored up in the wheel at time /,, 

 so that if the speed were then known M could at 

 once be calculated. But as we have no speed indi- 

 cator, we take it that the motion is uniformly 



accelerated till the cord drops off or we take 2— as 



the revolutions per minute at the time /,. The correc- 

 tions are of more interest. We have first to deduct the 

 kinetic energy of W when the cord drops off. Then 

 we must make experiments on the friction of the 

 pulley B, for the pull in the cord at C is less than 

 what it is at D, and these experiments are them- 

 selves very interesting, for they are made with the 

 two parts C and D vertical, so that the parallelogram 

 of force principle must be brought in to make them 

 available. Next we correct for the friction at the 

 pivots E and F. And here we observe that the aver- 

 age speed from f, to i. is the same as from O to 

 t|, and hence that from f, to i, the motion is uni- 

 formly retarded, and hence that there is as much 

 energy wasted in any one revolution as in any other. 

 If, then, we know the number of revolutions from 

 f, to f, we know the energy wasted in one revolu- 

 tion, and we can correct for friction before the cord 

 drops off, and so we make one correction after 

 another, and there is hardly any limit to the amount 

 of ingenuity required, as the corrections get less and 

 less important. I remember that four grey-headed 

 men worked together once at this piece of apparatus 

 in the evening for five weeks, and when at length 

 they had satisfied themselves with their corrections 

 they had practically used many times every important 

 principle of mechanics, and they had acquired a 

 handy working knowledge of all these principles." 



It is hardly possible to set bounds to the usefulness 

 of such calculations as the above in making students 



XO. 2015, VOL. 78] 



think for themselves and, if they have even a moderate 

 acquaintance with mathematics, in assisting them to 

 find themselves, even unwittingly, engaged on what 

 is really an original piece of research. 



A mechanics laboratory is by its constitution less 

 adapted than an engineering laboratory to research 

 work of the usual kind, but during the last few vears 

 an important piece of original work on the air fric- 

 tion of rotating paper discs has been carried out. A 

 note on the preliminary experiments was read before 

 the Southport meeting of the British .\ssociation in 

 1904 by Mr. W. Odell. Mr. Odell's experiments con- 

 sisted in measuring electrically the torque necessary 

 to keep in uniform rotary motion circular discs of 

 paper which were mounted on a horizontal shaft. 

 These discs were 15, 22, 27 and 47 inches in dia- 

 meter, and the torque was measured for (i) different 

 speeds, (2) different diameters. The verv interesting 

 result was found that, once the critical speed was 

 passed, the torque was proportional to the jith power 

 of the speed, and that n was about 25. It was further 

 found that the critical speed was roughly proportional 

 to the square of the diameter when different discs 

 were used. For a given speed the torque increased 

 with the S^th power of the diameter, and this striking 

 result leads to the deduction that, 6 watts only being 

 required to keep a 27-inch disc moving steadily at 

 550 r.p.m., no less than ^i h.p. would be necessary 

 to keep a g-feet disc rotating at the same speed. 

 This deduction has a verv important bearing on the 

 design of high-speed generators and other machines 

 in which the rotating parts have considerable 

 diameter. Roughly, a one per cent, increase in 

 diameter would lead to an increase in the necessary 

 torque for the same speed to be obtained of no less 

 than 55 per cent. 



A similar plan is followed in the laboratory with 

 regard to problems connected with the torsion of 

 shafts, the flow of water, the bending of beams, the 

 efliciency of mechanisms, the swinging of pendulums, 

 and others of the same kind. The result of such a 

 training on students is that they acquire a kind of in- 

 stinct in mechanical matters, one which is difficult to 

 describe, but which develops alertness, and would, 

 for instance, lead such a student to doubt im- 

 mediately the accuracy of the usual measurement 

 of horse-power bv means of the average indicator. 

 Even with such a well-made instrument as the Hop- 

 kinson reflecting indicator he would not omit to make 

 calculations as to the effect of inertia lag when 

 rapid explosions were being recorded, the effect of 

 damping, and other points. .\fter doing this he 

 would appreciate the more the modest claim of the 

 inventor of a 2 per cent, accuracy, and contrast it with 

 the far more heavily drawn claims of instruments 

 much less carefully designed. To instil this attitude 

 of mind into young students, that they should " test 

 all things " and take nothing for granted, is to lay 

 the basis of a scientific way of thinking which is of 

 fundamental importance to them in after vears. 



H. E. W. 



THE MILKY WAY.-" 



IT may be that the limitations imposed upon us by 

 restrictions in time and space will never allow a 

 complete solution of the problems offered by the study 

 of the sidereal universe. But the effort to comprehend 

 the processes that have contributed to its structure, or 



1 (1) " La Dislribution des Etoiles par rapport k la Voie lactiSe d'apres la 

 Cane et le Catalogue phr tographiques du Ciel." Par Paul Strcobant. 

 Extrait des AnnalVs de lObservatoite royal de Belgique, Annales a;trono- 

 miques. Tome xi.. Fascicule ii. 



(2) " Die Milchstrasse." By Prof. Ma.\ Wolf. Pp.48. (Leipzig :'. A. 

 liarth, igo8.) Price 4 marks. 



