Apeil 3, 1896.] 



SCIENCE. 



527 



laid is moving tlie decimal point till it stands 

 directly after the first significant figure. Thus 

 850.72=8.5072.10-; 0.000652=6.52.10-*. 



We cannot go into details, but may say that 

 Prof. Holman's rules are few and simple, and 

 so abundantly illustrated that students will 

 find little difficulty in applying them. The 

 book is probably the best, in its particular field, 

 which is available for American students and 

 engineers. When five-place tables are not suf- 

 ficiently accurate the author recommends the 

 well-known Vega or other seven-place tables. 

 It is a pity that engineers and others seem to 

 be unaware that Bremiker's six-place tables, 

 revised by Albrecht, are sufficiently accurate 

 for almost any problem which occurs in practice, 

 and are easier to use than any seven-place tables. 



A few peculiarities of Prof. Holman's book 

 deserve notice. Negative characteristics are 

 used, even in the tables, and recommended. 

 Decimal points are introduced in the arguments 

 of the tables of logarithms of natviral numbers ; 

 instead of 621, 6.21 is printed. Interpolation 

 tables are not given for all the tabular differ- 

 ences on a given page, when the differences are 

 large, even though there is ample room on the 

 margin of the page. The interpolation tables 

 given are not accurate. Thus 0.3 ■ 22 is called 7 

 instead of 6.6 ; this suffices in multiplying by 

 one figure, but in division needless inaccuracy 

 may arise. 



In the table of 5-place logarithmic trigonomet- 

 ric functions the argument is for each minute, 

 but no proportional parts are given. There is no 

 provision for finding accurately the logarithmic 

 sines and tangents of small angles involving 

 fractional parts of a minute. 



A student will sometimes wish that the au- 

 thor had been a little more particular in his 

 statements. On page xii., for example, after 

 stating two fundamental propositions, "which 

 one can easily verify by algebra or by numeri- 

 cal examples, ' ' the author adds : 



"A more general form of statement from 

 which these follow is : If several numbers are 

 multiplied or divided, a given percentage error 

 in any one of them will produce the same per- 

 centage error in the result. ' ' Take the example 

 i|fl = 60. The student will think that the 

 author means that if the divisor 2 be in error 



by 25^ of itself, the quotient is in error by 

 25 /o of itself. This he will find to be false. 

 Had the author given a definition of ' percentage 

 error, ' the student would be able to determine 

 whether the above statement were exact, or 

 siihply approximately true for such examples 

 as are likely to occur in practice. The two 

 propositions mentioned above might be im- 

 proved by re-writing. 



Two errata have been noticed: In the first 

 line of p. xxiii for ' numerator ' read ' denom- 

 inator;' in the last line of p. xii for 'merely' 

 read 'nearly.' 



The book is elegantly printed on heavy paper; 

 one can only wish that it were so bouud that it 

 would lie open with a flat page, a sine qua non 

 of logarithmetic tables. 



Herbert A. Howe. 



University of Denver. 



SCIENTIFIC JOURNALS. 



AMERICAN JOURNAL OP SCIENCE. 



The subject of the Eontgen rays is discussed 

 in the March number by A. A. Michelson, who 

 proposes a new hypothesis to account for 

 the phenomena observed. He mentions, first, 

 the two theories that have hitherto been sug- 

 gested, that of longitudinal waves and that of 

 projected particles, and remarks upon the special 

 difficulties which each of these theories meets. 

 His own hypothesis he calls the ' Ether- Vortex ' 

 theory, which he states as follows : 



' ' Let it be supposed that the X-rays are vor- 

 tices of an intermolecular medium (provi- 

 sionally, the ether). These vortices are pro- 

 duced at the surface of the cathode by the 

 negative charge, which forces them out from 

 among the molecules of the cathode. ' ' He shows 

 that certain of the phenomena which are most 

 typical and difficult to explain may be ac- 

 counted for on this supposition. The fact that 

 a high vacuum is essential within the tube 

 while, once outside, the rays can pass not only 

 through air, but also through many solids, is re- 

 garded as finding a solution if it be considered 

 that, in order that ether vortices may result from 

 the electrical impulse, this impulse must be com- 

 municated to them, and must not be dissipated 

 in the interchange of molecular charges which 



