NA TURE 



[Al'RIL 



1902 



Rotation of a Lamina Falling in Air. 



In your issue of iMarch 20, Dr. Johnstone Sloney, in reference 

 to ihe behaviour of ice spicula; in the clouds, instances the 

 spinninp of a card when dropped through the air. 



I thinU Lord Rayleigh was the first person to point out how 

 curious this phenomenon is and to show that the axis of a 

 spinning lamina might be held between beatings without aflect- 

 ing the result. Also that a lamina so held and placed in a 

 draught was equally ready to spin in either direction, thus 

 precluding the idea that the rotation might be due to some 

 want of symmetry in the lamina itself. 



A few years ago I made some experiments on the rotation of 

 laminre in air currents. 



The lamin;e were mounted in bearings as frictionless as I 

 could make them, and the experiment consisted (a) in measuring 

 the speed of the air and the angular rotation of the lamina, {h) 

 in mapping the How of the air past the lamina. This was done 

 by the use of smoke and intermittent illumination. 



It would take too long to describe the apparatus in detail, but 

 some of the results may be of sufficient interest for publication. 



To calculate the magnitude of the rrnple a prioti is, I 

 belitve, beyond the power of mathematical analy.sis at present. 



The experiments just alluded to were made with light rect- 

 angular lamina.', but these conditions are not essential— pennies 

 spin very nicely when dropped from a great height. 



I have not succeeded in making a lamina spin in a current of 

 water, probably because the densities of the Huid and solid are 

 not sufficiently difTcrenI, but if a flywheel were fixed on Ihe 

 axis of the lamina, so as to be out of the water, and thus- 

 increase the moment of inertia of the turning body without 

 altering the fluid friction, &c. . rotation might perhaps be 

 obtained in this case also. A. M.m.iock. 



March 22. 



Matheinatics and Science at Cambridge. 



To those who have at heart the advancement of scientific 

 knowledge in Great Britain it is impossible but to acknowledge 

 that the country owes a profound debt of gratitude to the Uni- 

 versity of Cambridge. During the dark ages of education her 

 colleges formed llie stronghold of modern culture ; and the 



Thus, the angular velocity of the lamina was found to be 

 (through a considerable range of air speed, v, and breadth of 



lamina, b) = constant x-^. 

 b 



There is a limit, however, to the smallness of the lamina 

 which will revolve (e.g. small lamince of gold-leaf a tenth of 

 an inch long and two or three hundredths broad do not re- 

 volve, but oscillate as they fall). 



The cause of the rotation depends on the way in which the 

 eddies are formed on the downstream side of Ihe lamina. 



The changes which occur, in Ihe eddy making, during half a 

 revolution are shown in the accompanying .sketches. 



From these it will be seen that Ihe spin causes the air to flow 

 unsymmetrically with regard to Ihe axis of Ihe lamina, and that 

 there will be, on the whole, a transverse force on the axis lend- 

 ing to press it in Ihe direction of rotation of Ihe leading edge. 



It can be seen also that before the lamina reaches Ihe positions 

 I or 7, there will be a couple acting on it lending to accelerate 

 ihe lolation, and that Ihe couple will certainly be positive from 

 position I to 3. It may be negative from between positions 

 3 and 4 to 6 ; but at any rate for more than half Ihe total 

 time the rotation is being accelerated. 



NO. 1692, VOL. 65J 



principle was there maintained that to the students of dead 

 languages, however paramount they might be without her walls, 

 within them no more than equal rights should be conceded. 

 This involved a contest in which the principals were Ihe mathe- 

 maticians and classics, and the terms of peace a system of 

 specialisation coupled with a certain test examination, an in- 

 quisitorial farce known as Ihe Lillle-Go. This system, 

 adopted doubtless in order to obviate Ihe difficulty of forming a 

 combined curriculum of studies which appealed to totally dis- 

 similar taste-s, has since been so widely extended in its applica- 

 tion as to divorce subjects which are germane in character and 

 naturally adapted for a close alliance. The climax is reached in 

 Ihe ever-growing barrier which separates mathematics from 

 natural science, and which is year by year made more impas- 

 sable by Ihe examinations for entrance scholarships. The.se are 

 of high value ; and without this aid from Ihe funds which the 

 college authorities hold in trust for educational purposes many 

 a brilliant man would be unable to consummate his work at 

 school by a university career. It is, therefore, necessary that 

 school curricula .should be arranged in stricl .accordance with 

 their requirements. 



In view of their future careers, .schoolboy mathematicians are 

 divisible roughly into three classes — those who will make a pro- 

 fession of mathematics, those who will apply mathematics to 

 science, and those who will, after obtaining .academic honours, 

 entirely sever their connection with the subject. To the last of 

 these the question of syllabus is of comparatively small im- 

 portance. i?ut to the first and second it is of the utmost im- 

 portance, and especially to Ihe second, as it is essential to a 

 man who for several years studies mathematics with an ulterior 

 object in view that, while he should acquire a thorough grasp of 

 the principles of Ihe subject, he should not be forced by ex- 

 amining bodies to apply himself to what can only be regarded 

 as intellectual pastimes ; nor even in the case of the first can 

 great skill in work of this class \m\V pari passu with a grasp of 

 principle. 



