286 
compared with facts, the terms to be used are “hypo- 
thetical” and “practical.” 
This is one of three advertised books of ‘“ The 
Specialists’ Series” which deal with electric engineering. 
Another of the three is devoted to the subject of magneto- 
and dynamo-electric machines, and the third is on electric 
lighting. We think it probable that in the greater part 
of Mr. Kapp’s book he is going over ground which belongs 
almost altogether to the author of one of the other books 
of the series. As Mr. Kapp treats his subjects well, 
however, we cannot much object to this ; but what we do 
object to is, that while taking up the subjects of the other 
authors, he has not given us his own subject. 
three pages, or about one-fifth of the book, an instructive 
account is given of the various attempts which have been 
made to drive carriages on railways, telpher lines, ploughs, 
cranes, fans, and pumps, and we understand from: Mr. 
Kapp’s introduction that it is to this sort of transmission 
of energy that his book is devoted. 
Now it is not merely sufficient for the author to give 
an account of what has already been done in this wav ; 
the reader expects a correct theoretical treatment of the 
whole subject, the cost of conductors, the fall of potentia) 
along the conductor, and the efficiency of transmission. 
These questions are sufficiently well taken up fora treatise 
on electric lighting, but for a book on the electric driving 
of machines at a distance the subject can hardly be said 
to be touched upon. Thus, for example, the develop- 
ment of Sir Willian Thomson’s law as applied in electric 
light installations, and published by Prof. Forbes in his 
lectures at the Society of Arts, is carefully given. Now 
small alteration of potential difference at an incandescent 
lamp may produce disastrous effects on the lamp, may 
destroy it, or may cause sudden darkness, and this is the 
most important consideration in arranging conductors 
for lighting purposes ; whereas, in the electric driving of 
trains or machinery, small alterations of potential differ- 
ence are of no importance whatsoever. In consequence 
of this, in driving machinery electrically there may be a 
very considerable fall of potential along the conductor 
from the dynamo to the motor, and hence motors may be 
worked directly at distances which it would be absurd to 
contemplate in working an incandescent lamp. In fact 
the question of cost of conductors must be treated from 
quite a different point of view in the two cases, and it 
seems to us that Mr. Kapp has taken up the point of 
view which is most remote from his subject. 
We think Mr. Kapp’s book a very valuable addition to 
electrical engineering literature. It will be widely read, 
and it deserves the popularity which it will receive. Had 
we not thought it to be so excellent in many ways we should 
not have criticised it so narrowly, and, in spite of our 
warning to the student, we are very glad to meet with 
originality in leading up to the theory of the dynamo- 
machine. We are glad to see that the author has slightly 
amplified his account of the method, now in general 
use, of calculating the probable electromotive force 
of a dynamo-machine, which he published in his 
paper. The method is known to be practically correct, 
although it is based on a magnetic hypothesis of which 
there is no recognition in any book on physics—the 
hypothesis of magnetic resistance. We could have 
In sixty- | 
NATURE 
[Huly 29, 1886 
thesis, as we know of no actual results of experiment 
having yet beea published which give it a general verifi- 
cation. 
In reading over this criticism we feel that our 
objections to the book have all been brought very pro- 
minently forward. It would be very easy to point out 
here much that is good in the book, but perhaps our 
readers would then find this article long and tedious. Any 
reader of the book will find original and interesting views 
in every chapter ; it is not every reader who would for 
himself have noticed the faults which we have here 
gathered together. We have achieved the difficult task 
of finding fault with an excellent book. 
JOHN PERRY 
OUR BOOK SHELF 
The Aryan Maort. By Edward Tregear. 
N.Z.: George Didsbury, 1885.) 
THIs little book contains a theory that the ancestors of the 
New Zealanders belonged to the Aryan race, and were a 
pastoral people. To signify this, the cover is adorned with 
a golden picture, seemingly representing a Maori warrior 
in native guise, accompanied by a sturdy little Highland 
bull. Now, it being notorious that the New Zealanders, 
when discovered, had no cattle nor remains of them in 
their country, the reader’s curiosity is aroused to see how 
Mr. Edward Tregear supports this unlikely thesis. His 
method proves to be a philological paradox which we 
have never met with before. For example, it is argued 
(p. 31) that the Maoris once knew the bull by a word like 
the latin fawrus, a bull. How so, one asks, when they 
(Wellington, 
' no more had the word in their vocabulary than the beast 
on their land? The answer is, that in the absence of the 
word ¢aurus itself the author relies on a dozen or so of 
other Maori words which he alleges to refer to it. The 
following are a few of them:—Zara, had courage ; 
tararau, made a loud noise ; ¢avarva, had two points or 
peaks ; taveha, was red; ¢farehu, caught one unawares; 
zarore, had a noose put on him; Zaruke, lay dead in 
numbers (if it was characteristic of the bulls to lie dead 
in numbers, how multitudinous the cows and calves must 
have been in the Aryan-Maori herds !). The poverty of 
the Maori language in consonants makes it easy to the 
author to play this fanciful game with his dictionary to 
his own full satisfaction. He takes a real interest in 
studying the Maoris, and though he has gone astray this 
time, he may, if a young man, do something more worth 
doing in the collection of native customs, legends, games, 
and the like which the older natives still remember. 
LEDBLERS LO PAE EDITOR: 
[Zhe Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents. Neither can he undertake to 
return, or to correspond with the writers of, rejected manu- 
scripts. No notice ts taken of anonymous communications. 
[The Editor urgently requests correspondents to keep their letters 
as short as possible. The pressure on his space is so great 
that it ts impossible otherwise to insure the appearance even 
of communications containing interesting and novel facts.] 
Tidal Friction and the Evolution of a Satellite 
In NaTuRE, vol. xxxiii. p. 367, is an article by Mr. G. H. 
Darwin, defending his theory of tidal evolution, and dealing 
with what I have written respecting that theory. Space will 
here prevent my replying at length to the above ; but as the 
author of it seems to think that my inquiry has been confined 
too much to the mode of origin of the moon, I have pushed it 
out in other directions, when important results have been 
obtained. I purpose here chiefly devoting my space to these, 
wished that Mr. Kapp had dwelt more upon this hypo- | which can be put in a comparatively short and simple form, 
