684 
OUR BOOKSHELF. 
Die vradioaktive Strahlung als 
wahrscheinlichkeitstheoretischer Untersuch- 
ungen. By Prof. L. v. Bortkiewicz. Pp. 84. 
(Berlin: Julius Springer, 1913.) Price 4 marks. 
Tuts mathematical work is a critical applica- 
tion of the theory of chance to the breaking down 
of radio-active atoms. Its discussion is mainly 
based on the experiments of Rutherford and 
Geiger. Scintillations were produced on a screen 
by polonium, and were counted over a succession 
of equal short intervals of time, and the intervals 
were classified by the number of them which 
showed either no scintillation or one or two 
or more. The experimenters found that their 
numbers. agreed well with those predicted by 
the theory of pure chance, but they gave no 
criterion as to the closeness of agreement to 
be expected. The calculation of the “mean 
errors”? is a simple matter, but in the com- 
parison of such a series of numbers it is only likely 
that in a few of the cases the mean error should 
be considerably exceeded. Prof. Bortkiewicz 
therefore provides a single test for the whole ex- 
periment. He works out twelve cases, and con- 
cludes that the results are, on the whole, slightly 
closer to their most probable values than is pre- 
dicted by theory. He suggests an experimental 
cause for this small discrepancy. He also 
discusses one of the experiments of Marsden and 
Barratt, who made their analysis by classifying 
the lengths of time between each two successive 
scintillations, and he concludes that the distribu- 
tion isnormal. In this case his test is not perfectly 
satisfactory, as it involves the use of quadrature 
and interpolation formule, processes which would 
seem to be very unsuitable for problems of chance. 
In both types of experiment distributions can be 
contrived which pass his tests, and yet are in 
reality very improbable, but no doubt there are 
great mathematical difficulties in the way of deriv- 
ing the true probability test. From his work we 
may conclude that the search for regularity, other 
than the regularity of chance, in the disintegration 
of radio-active atoms is not a hopeful quest. 
CG Dp: 
A Pocket-Book for Miners and Metallurgists: 
Comprising Rules, Formule, Tables and Notes 
for use in Field and Office Work. Compiled by 
F. D. Power. Third edition, corrected. Pp. 
xiv+371. (London: Crosby Lockwood and 
Son, 1914.) Price 6s. net. 
MINING engineers are nowadays called upon for 
knowledge and powers in so many directions that 
to anticipate moderate success and escape serious 
blame, they must exhibit qualities for which Gilbert 
and Sullivan’s heavy dragoon could not hope. To 
be ready to act at short notice as an explorer, a 
geologist, a civil and mechanical engineer, a 
chemist, a metallurgist, a doctor, and a lawyer, 
a man needs some little book in his pocket which 
he can consult as each new problem comes into 
view. - Such a book Mr. Danvers Power set him- 
self to construct many years ago, and the third 
NO. 2378) VOR. G2] 
Gegenstand 
NATURE 
[FEBRUARY 19, I914 
edition, now issued, is not less successful than 
its forerunners. There is no trace of the amateur 
about the little volume. “It is the work of a 
professional man who has set down the things he 
wanted to know himself. Like all pocket encyclo- 
peedias, it does not contain everything that could 
be wished for. There might have been included 
something about furnaces, refractory substances, 
and melting points, a few tips on mine-surveying 
problems, a little more about the strength of 
materials, and perhaps some information on first 
aid. But although there may be a few omissions, 
so much is included that the book deserves a trial 
by every prudent miner or metallurgist. 
LETTERS TO THE EDITOR: 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he wndertake to return, or to correspond wilh 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications.] 
The Constitution of the Interior of the Earth as 
Revealed by Earthquakes. 
On p. 45 of Dr. G. W. Walker’s recently published 
book, ‘‘Modern Seismology,’ I find the following 
sentence :—‘It has sometimes been asserted that S 
never reaches beyond a certain distance, and to ex- 
plain this an impenetrable core of the earth has been 
assumed. We see that no such hypothesis is at all 
necessary to explain the observations.”” The reference 
here seems to be to a paper, by myself, *‘The Con- 
stitution of the Interior of the Earth as Revealed by 
Earthquakes,’ which was published in the Quarterly 
Journal of the Geological Society (vol. Ixii., 1906), or, 
more probably, to the references to this ypaper con- 
tained in Prof. Wiechert’s paper, ‘‘ Ueber Erdbeben- 
wellen,”’ published in the Nachrichten d. K. Gesell-— 
schaft d. Wissenschaften (Gottingen, 1907), and as the © 
summary dismissal of the subject indicates an imper-— 
fect appreciation of the problem, which is one of the 
important problems of the immediate future of seismo- 
logical research, I trust you will afford me space to 
state the position. 
In my paper, referred to above, I pointed out that — 
the twofold character of the preliminary’ tremors, 
representing the arrival of two distinct forms of 
wave motion, can be traced continuously up to a 
distance of about 110° or 1200 km. from the origin, 
and that a comparison of the times of arrival of the 
waves at different distances shows a progressive and 
gradual increase of interval with distance, and affords 
no indication of any great change in the character of 
the material traversed by the wave paths. Beyond 
12,000 km., however, the second phase can no longer 
be recognised with certainty, and has either entirely 
disappeared or is represented very feebly and-with a 
considerable delay, as compared with the time of 
arrival which would be anticipated from the records of 
observations at lesser distances from the origin. From 
this I concluded that the wave paths to these more 
distant stations must have entered a central core of 
matter differing markedly in constitution from the 
outer portion of the earth, in that it was either quite 
incapable of transmitting the second-phase waves, or 
only transmitted them’ with a considerable diminution 
of energy and of rate of transmission. 
Prof. Wiechert explains the facts in a different 
manner. From the laws of reflection, and assuming 
a tolerably homogeneous earth, he deduces the con- 
