Mat 28, 1915] 



SCIENCE 



793 



in the world, and the section sent by Ameri- 

 can scientific men last year sufficiently demon- 

 strated the place held by this country in ap- 

 plied photography. It is very desirable that 

 American scientific photography should be 

 equally well represented in 1915, and, in order 

 to enable this to be done with as little difficulty 

 as possible, I have again arranged to collect 

 and forward American work intended for the 

 scientific section. 



This work should consist of prints showing 

 the use of photography for scientific purposes 

 and its application to spectroscopy, astronomy, 

 radiography, biology, etc. Photographs should 

 reach me not later than Thursday, July 1. 

 They should be mounted but not framed. 



I should be glad if any worker who is able 

 to send photographs will communicate with 

 me as soon as possible so that I might arrange 

 for the receiving and entry of the exhibit. 



C. E. K. Mees 



Research Laboratory, 

 Kodak Park, 

 eochester, n. y. 



SCIENTIFIC BOOKS 



Report on Gyroscopic Theory. By Sir George 

 Greenhill. Eeports and Memoranda, No. 

 146, Advisory Committee for Aeronautics. 

 London, T. Fisher Unwin, 1914. Pp. iv + 

 278, with 49 illustrations. Price 10 shillings. 

 Many people wonder at the expenditure of 

 time and energy given by the mathematician 

 to subjects like the theory of groups and differ- 

 ential equations. Others can not understand 

 why men of the ability of Klein, Perry and 

 Crabtree should lecture upon the theory of the 

 top. Still others fail to see in the studies 

 made by Maxwell of his spinning top in an agate 

 cup, or of Sommerfield and Noether on the 

 gyroscope, anything to justify a student in 

 following in their footsteps. And yet, when 

 we reflect that the spinning top illustrates a 

 group of motions, that its theory involves the 

 differential equation at the very outset, that 

 the earth is merely a moderate-sized top spin- 

 ning in space, that the solar system is a some- 

 what larger one, and that many nebulse are 

 solar systems in formation, the subject assumes 



a different aspect, even to the man in the 

 street. And when he further reflects that the 

 stabilizing gyroscope, now made in large num- 

 bers by Sperry's company, is used on the 

 aeroplanes above the iiring lines in the great 

 war, and acts as a literal balance wheel on the 

 super-dreadnoughts of the warring powers and 

 can be bought in the offices of the makers in 

 any of the large capitals of the world, this 

 same man in the street begins to see that the 

 theorist may touch upon the very practical 

 and that the practical man may well afford to 

 look to the man of theory for help in the affairs 

 of the real life of the present day. 



It is such popular considerations as these 

 that may weU. lead the man of dollars to wel- 

 come, even if he can not understand, a monu- 

 mental treatise like this which Sir George 

 Greenhill, with his usual modesty, has called 

 a simple report. To the general man of sci- 

 ence the work will mean much more, even if 

 he too shall fail to read 278 large quarto pages 

 devoted chiefly to mathematics. But to stu- 

 dents of analytical mechanics, and particularly 

 to those who look for applications of modern 

 mathematics to dynamics, the work will stand 

 as a monument of patient research on the part 

 of a man who works con amore and with an 

 extended vision in a field of rapidly increasing 

 importance. 



Sir George Greenhill always writes as he 

 talks, and he never talks like the man whom 

 he delights to refer to as " a mere mathe- 

 matician." As he sits at the head of a work 

 table in his quaint room in Staple Inn — ^the 

 room in which Dr. Johnson may have written 

 Easselas — and talks of his labors on the gyro- 

 scope, he is a mathematician for about a 

 minute, a man with the zeal of a boy for an- 

 other minute, a charming raconteur of stories 

 of his master, Maxwell, the minute later, and 

 an appreciative student of his friends Klein 

 and Sommerfield in the next unit of time. 

 And this description characterizes his ad- 

 dresses, his books, his memoirs and his re- 

 ports — they are all human, the product not 

 merely of the mathematician, not merely of the 

 student of dynamics, not merely of the experi- 

 menter in the laboratory, but always of the 

 big-hearted man. 



