Fepruary 18, 1897] 
NATURE 
365 
taken as the necessary basis on which to build up the 
complete picture of such region as the sphere of human 
activity.” A book constructed on these lines claims 
attention at the outset ; and when, as is the case with the 
volume before us, the pages give evidence that the author 
is thoroughly familiar with all the geographical facts 
pertaining to the region with which he deals, we have 
the factors which combine to make a work useful as an 
educational instrument, valuable for reference, and 
interesting to geographical readers. It is, indeed, not 
too much to say that no book now in existence contains 
within such a small compass so much accurate informa- 
tion on the African continent as is given in Mr. 
Heawood’s little volume. The book should be widely 
used in schools, and for this purpose the summary of the 
geography of Africa will be found very serviceable. 
To every one who wishes to possess a concise statement 
of the physical features, native inhabitants, history, and 
political development of Africa, the volume can be con- 
fidently recommended. 
Crags and Craters: Rambles in the Island of Réunion. 
By W. D. Oliver, M.A. With illustrations and a map. 
Pp. xiv + 213. (London : Longmans, Green, and Co., 
1896.) 
IF we were going to Réunion (Bourbon), or had lately come 
back from it, we should be very glad to fall in with such 
a book as this. It gives an account of the experiences of 
an energetic man who spent six months on the island, 
and went about wherever curiosity led him. There is a 
good map and several photographs. The only illustra- 
tion that is not a photograph is wretchedly bad. Our 
author writes easily and clearly, and has evidently taken 
pains to collect plenty of detailed information. Here 
the reader finds geography, history, statistics, scenery, 
manners and customs of the people—almost everything 
that can be desired, except natural history. What a 
pity that Mr. Oliver did not inquire beforehand what the 
naturalist wants to know about Réunion! Prof. Newton, 
of Cambridge, would have put him in the way of doing 
some really good work. A little fresh information about 
the extinct, or nearly extinct, birds, and the gigantic 
land-tortoises (if there are any in Réunion) would have 
greatly enriched the book. In spite of this deficiency, 
“Crags and Craters” is a valuable contributicn. The 
schoolmaster in search of graphic details about the 
islands of the Indian Ocean would find much good stuff 
here. rc. M. 
Everybodys Guide to Photography. By “Operator.” 
Pp. 162. (London: Saxon and Co.) 
AMATEUR photographers are now so very numerous, 
that this book should find a large number of readers. 
There are hints on the choice of a photographic outfit, 
and simple directions on all the operations concerned in 
the production of good negatives and prints. Instruc- 
tions are also given how to make enlargements and 
lantern slides, and on the use of orthochromatic plates, 
the production of stereoscopic photographs, flash-light 
photography, and Réntgen ray pictures. 
Ls Natural Selection the Creator of Species? By Duncan 
Graham, Pp. xviii + 303. (London: Digby, Long, 
and Co.) 
ACCORDING to the author of this book, evolution by 
natural selection is a snare and a delusion. Wherefore, 
he comes forward to sweep away the whole fabric of 
evolution, and*to show “that the condition of the earth 
and ‘its inhabitants cannot be explained by the action of 
physical forces, independent of support and direction 
from an intelligent power.” His qualifications for this 
task may be judged from the avowal that, although he 
has studied the nature and habits of animals and plants 
for many years, he has never discovered evidence that 
conclusively indicated evolution. 
NO. 1425, VOL. 55 | 
LETTERS TO THE EDITOR. 
(Zhe Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents. Netther can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts intended for this or any other part of NATURE. 
No notice ts taken of anonymous communications. ] 
The Force of a Ton. 
THE hydraulic forging presses at the Armstrong works, which 
I had the privilege of visiting a short time ago, bore the inscrip- 
tions—2000 tons—5000 tons; meaning ihereby the thrust 
exerted by the ram; and Dr. Lodge’s opinion that the word 
wezght should be supplied was rejected by the engineers, as the 
addition of the word weggh¢ would imply that the presses werghed 
2000 or 5000 tons. 
It is quietly assumed by Dr. Lodge and his followers that the 
word wezgh¢ is never used except to denote the force with which 
a body is attracted by the Earth ; as if we should never translate 
weight by pondus, un potds, gewicht, but always by grave, un 
grave, -schwere; as in Galileo’s memoir ‘‘ De motu gravium 
naturaliter accelerato.” To support this assumption the Act 
of Parliament on Weights and Measures is always quoted in a 
garbled form, with a view of making out that the standard 
Pound Weight is really not the lump of platinum specified in the 
Act, but the pressure on the bottom of the box in which it is 
preserved ; probably with the mental reservation of the Mikado, 
‘*this is the careless way in which the Act is drafted ; we will 
have it altered next time.” 
Thus the weight of the standard pound weight should, in 
Dr. Lodge’s language, be given as 32°1912 poundals, when at 
rest in the box at Westminster, and when it is high-water at 
London Bridge, but changing suddenly to about 32°2382 
poundals when tossed in the air. What the thrust of 5000 tons 
would become when expressed in poundals, funals, or even 
tonals, it is fearful to contemplate, as well as the pressure of the 
water in the press or modern steam pressures in poundals on the 
square foot. 
If this controversial question is studied historically, it will be 
found that Prof. Perry is quite right in maintaining that the 
quantity denoted by #z in Dynamics, and called the mass, is 
measured in units of zverfza; the unit of inertia being that 
quantity of matter which receives unit acceleration from the 
unit force. 
In all continental treatises, and in our own engineering works, 
the quantity w/¢ is replaced by the letter 7 and called the mass ; 
this defines the unit mass as that quantity of matter which will 
receive the unit of acceleration from the gravitation unit of 
force. 
Dr. Lodge changes to the absolute unit of force, and now 
replaces mz by w; so that if the mass of a body is # pounds, it 
must weigh 7z lb. ; and if moving with velocity wv //s, its kinetic 
energy is sv*/2 foot-poundals, or mz'/2g foot-pounds ; we have 
now come back to the engineer’s measurement, except that his 
w has, for some mysterious reason, become 7, and a different 
to his w/z. 
I agree with Mr. C. S. Jackson, to a certain extent in oppo- 
sition to Prof. Perry, in the opinion that the substitution of 
for w/g had better be abandoned ; or, asacompromise, the letter 
m may replace w ; because a body whose mass is 7 or w pounds 
must weigh 7 or w 1b. in the balance ; in ordinary language, its 
weight is #z or w Ib. 
It is the old medizeval discussion of Nominalism and Realism 
over again ; does the thing alter when we call it by a different 
name? If a steamer loads 1000 tons of coal, are we no longer 
to say that this coal weighs 1000 tons; or that 1000 tons weight 
has been placed on board? Are we to be compelled to say 
that the coal masses 1000 tons ; and that it is 1000 tons mass ? 
In a redetermination of the volume of the gallon, Mr. H. J. 
Chaney has found that a cubic inch of distilled water, freed from 
air, and weighed against brass weights in air, when the tem- 
perature is 62° F. and the height of the barometer is 30 inches, 
is equilibrated by 252°286 grains ; and this makes the volume 
of the gallon 277°463 cubic inches, according to the Act of 
Parliament (PAz/. TZrans., 1892). Mr. Chaney calls’ this 
252°286 grains the mass of a cubic inch of water ; but if the 
same weighing is carried out in vacuo, according to another 
clause of the Act, an extra 0266 grain must be added to 
maintain equilibrium ; what are we now to call this 252°552 
grains, with respect to a cubic inch of water? 
