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NATURE 
| DECEMBER 6, 1906 
comes patent that Hansen was a master of beautiful 
mathematical device. The bewildering detail ceases to 
obscure it when put into arithmetical form. 
The problems of exact solution of the secular per- 
turbations and of finding integrals of the equations of 
motion, which hold good for indefinite spans of 
time, must in the future occupy more and more atten- 
tion. 
Lately Hill has published an important memoir 
upon the subject (Astronomical Journal, vol. 
xxv.); two papers, Nos. 37 and 47, in the present 
volume show that his interest is not of recent date. 
No. 41, ‘‘ Reply to Mr. Neison’s Strictures on Delau- 
nay’s Method,’’ shows him as a critic—a formidable 
though reserved antagonist not without ironic humour. 
No. 44, ‘‘ On the Interior Constitution of the Earth as 
Respects Density,’’ is a beautiful example of what he 
can do when working without the fetters of exact 
astronomy. Eventually the paper is a solution of the 
well-known equation for the density of a spherical 
aggregation of gravitating gas at constant tem- 
perature. The question had been treated before, but 
Hill’s method is out of all measure more striking and 
complete than had been given previously. 
NEOLITHIC MAN. 
Neolithic Man in North-East Surrey. By Walter 
Johnson and William Wright. With a chapter on 
Flint by B. C. Polkinghorne. Cheaper re-issue. 
Pp. viiit+200. (London: Elliot Stock, 1906.) Price 
3s. 6d. net. 
HIS book is the result of several years of 
archeological investigation in the north-east 
corner of Surrey. The area visited measures about 
143 miles by 13 miles; it would fall between the 
Thames on the north and a line drawn between Box- 
hill and Oxted on the south. Within these limits the 
researches of our authors have been patient and un- 
wearying; they have sought for traces of Neolithic 
man in field after field, on height after height. Set 
down in their pages is a large amount of information 
as to his homes (Worms Heath, Croham Hurst, 
Barrow Hill, &c.) and burial-places; as to his methods 
of work, agricultural and domestic; as to the food 
he ate and the implements he used, celts, hammer- 
stones, arrow-heads, scrapers, &c. Some space too 
has been devoted to his track-ways and fortifications ; 
in most cases the same ground was occupied at a 
later period by Roman roads and works, or those of 
other invaders less skilled in engineering. 
The main subject is prefaced by an account of the 
various inhabitants who have succeeded one another 
in this country, especially the Neolithic and bronze- 
using peoples, and by a survey of the geological 
features of Neolithic Surrey. Our authors are 
certainly right in holding that the “ages’’ over- 
lapped or merged into one another; the terms ‘‘ Stone, 
Bronze, Iron age’’ are, in fact, merely conventional ; 
they can only be applied to the phases in develop- 
ment during which stone, bronze, or iron began to 
be worked, side by side with the material already in 
use, not of necessity replacing it. In Lancashire, 
NO. 1936, VOL. 75| 
for example, where the hills to the north pre- 
vented ready retreat in that direction, stone, 
bronze, and even iron seem to have been used con- 
temporaneously at one period (see ‘‘ Victoria County 
History of Lancashire ’’). Generally speaking, these 
opening chapters are highly instructive and accurate ; 
they form a useful introduction to the story of Neo- 
lithic man. If they have a fault it is that they are 
perhaps a trifle too technical. Being a cheap re- 
issue, the present edition is obviously intended for 
the local student, to whom—not, of course, to the 
specialist—this preliminary information might be ex- 
pected to be of service. In these circumstances the 
authors might have done better to leave out words 
like artifact, homotaxial, &c.; even geological terms 
like patina and Pleistocene, or at any rate to give 
their meaning. To leave them unexplained is to pre- 
suppose knowledge which is only too often to seek. 
We have neither the space nor the necessary local 
knowledge to enter into a detailed criticism of the 
main subject. If we may venture a suggestion, we 
should recommend greater caution in the use of 
arguments based on etymology. We doubt very 
much whether all the authors’ results (e.g. in 
chapter x.) would stand the scrutiny of a trained 
philologist. _ Only those who have made a special 
study of place-names are able to realise how danger- 
ous and misleading this kind of evidence is apt to be. 
There can be no question that the book is a valuable 
one. The extent of general knowledge displayed in 
it, and its high standard of scholarship, place its 
authors far above the ordinary run of local archzo- 
logists. Their work is only popular in the sense 
that it is inexpensive. In addition to the maps and 
illustrations by Sidney Harrowing and Frank Percy 
Smith, it has an index and a list of the authorities 
referred to in each chapter. Mr. B. C. Polking- 
horne contributes a supplementary chapter on the 
constitution and alterations of flint, with reference to 
the subject of flint implements. 
OUR BOOK SHELF. 
In the Days of the Comet. By H. G. Wells. Pp. 305. 
(London: Macmillan and Co., Ltd., 1906.) Price 6s. 
TuouGu the actual collision of the earth with the head 
of a comet is an extremely improbable event, it is 
not beyond the bounds of possibility. In 1861 the 
earth passed through the tail of a comet; and the 
showers of meteors occasionally observed near the 
end of November are probably due to encounters with 
fragmental remains of Biela’s lost comet. The disc- 
like appearance of Holmes’s comet in 1892 gave rise 
to the suggestion that the comet was approaching the 
earth head-on, and we believe Mr. Wells then used 
the idea in one of his clever short stories. In any 
case he would have no difficulty in finding justifica- 
tion for the supposed collision with a comet which 
forms the deus ex machind of the present romance. 
The comet which springs from Mr. Wells’s imagin- 
ative brain is seen in its early days by an enthusiastic 
amateur astronomer who forms one of the minor 
characters of the story as a “‘ quivering little smudge 
of light among the pin-points,’”? while the spectroscope 
showed ‘‘ an unprecedented band in the green.” The 
unknown element which this peculiar green radiation 
