JANUARY 3, 1907 | 
NATURE 221 
like surface. For ordinary moist climates, however, 
these tests serve as a clear condemnation of all lime- 
stones. Even the gritty Kentish Rag (p. 45) comes 
out badly, though, in combination with the ferru- 
ginous sandstones of our Lower Greensand, it has 
been known to make a road that held well together in 
dry seasons. 
The question of composite roads would be an inter- 
esting study in itself. Materials showing great 
differences under the attrition-test should, of course, 
not be used in association; but roads made of mixed 
gravel taken out of streams show good results in many 
parts of Europe. Similar material is usefully sup- 
plied by the glacial gravels nearer home. Teachers 
of practical geology, as well as all county and borough 
surveyors, will be grateful to the three authors for 
providing a remarkably cheap, clear and thoughtful 
treatise on a subject that the whirligig of time has 
again made of national importance. (Ge INS No Ce 
DYNAMO DESIGN. 
Elementary Principles of Continuous Current Dynamo 
Design. By H. M. Hobart. Pp. x+220. (Lon- 
don: Whittalker and Co.) Price 7s. 6d. net. 
ie is scarcely necessary at this date to recommend 
a book by Mr. Hobart on the design of direct 
current dynamo machines; it is safe to say that any 
production by this author will repay the study of 
practical men, and the present book forms no excep- 
tion. 
The contrast presented between a volume setting 
forth the results of the practical experience of a. man 
engaged in actual worl and a bool evolved out of 
the inner consciousness of a man who has access only 
to the theory of the subject is very striking. Books 
of the former class are comparatively rare, and are 
correspondingly valuable. 
Dealing in a general way with Mr. Hobart’s work, 
the first point that strikes -one favourably is the 
emphasis laid on the necessity of a large amount of 
application on the part of the student of the prin- 
ciples and methods set forth. These principles and 
methods must be regarded as the framework on which 
a designer is to build; and it is folly for him to 
assume that he is acquainted with the subject unless 
and until he has gone a long way in completing the 
structure by his own labour. The value of the book 
lies in the essential soundness of this framework, 
more particularly of the fundamental ideas on which 
it is itself based than on the framework itself. The 
commercial point of view is not instinctive with de- 
signers, and it is of the greatest importance that it 
should be acquired as soon as possible. For this 
reason Mr. Hobart has done well to lay stress on 
the necessity of judging every design by taking into 
account its first cost as well as its technical merits. 
Regarded in this way, the bool: consists of a series 
of statements explaining the way in which a dynamo 
should be considered as a successful machine or the 
reverse, and of a short account of several methods 
whereby the designer may himself estimate the first 
cost. 
NO. 1940, VOL. 75 | 
After preliminary chapters on what may be called 
the practical theory of the continuous current dynamo, 
Mr. Hobart deals at length with those considerations 
which form the limits in the design, namely, heating, 
sparking, and efficiency. Numerous constants and 
formulae are given, and miscellaneous information 
from which efficiencies can be calculated. The spark- 
ing data are, naturally, based on the method of react- 
ance voltage, introduced some years ago by the author 
and Mr. Parshall, although a long list of references 
is given to those who have contributed to the theory 
in recent years. This method, or some modification 
thereof, is so widely used that there is no necessity 
to describe it here. The constants for dealing with 
the heating and the efficiency are, perhaps, the least 
praiseworthy part of the book, or rather not so much 
the constants as the general method. The treatment 
in both cases seems somewhat arbitrary; for instance, 
it is not absolutely certain that the rise in temperature 
of the armature is proportional to the total watts lost— 
copper plus iron loss—-divided by the area of the 
cylindrical surface. Again, the method of estimating 
the iron loss in the armature is distinctly rough. This 
point has been debated at considerable length in the 
columns of the technical Press; but in the present 
writer’s opinion there are other methods which cer- 
tainly give better results. The calculation of the 
bearing friction and windage is referred to a single 
curve giving the relation between this loss and the 
value of D*L at the speed of tooo revolutions per 
minute; but there seems to be no indication as to how 
the loss varies with the speed, whether in direct pro- 
portion or as the 1.5th power of the speed. 
These slight discrepancies somewhat diminish the 
value of the book as a work of reference; but the 
essential feature of the book consists, as already 
stated, in the enforcement of a general grasp of the 
whole problem, commercial as well as technical. 
The book contains a large number of tables in which 
the various calculations are set out; some are filled in 
and others are left blank for the convenience of the 
student. It will thus be seen that this is a work 
which can be thoroughly recommended to the student 
and the designing engineer alike. 
OUR BOOK SHELF. 
Irrigation with Surface and Subterranean Water; 
and Land Drainage. By W. Gibbons Cox. | Pp. 
Vili+ 297; illustrated. 
son, 1906.) 
Tue author of this book has been engaged for many 
years in Australia in water supply and irrigation 
works. There are vast areas of land in that country 
the soil of which is of the highest fertility, but is 
barren and comparatively useless because of periodical 
aridity. The problem of irrigation of the land from, 
the rivers and creeks that flow at times through 
these districts, and form inexhaustible accumulations 
of underground water, is treated fully and practically 
according to the latest and most approved methods. 
_ With all its natural wealth and resources Australia 
is subject to the great drawback of occasional 
droughts of greater or less severity. The conse- 
quences of one of these droughts is thus graphically 
described :—‘‘ The natural water supply of the dis- 
(Sydney : Angus and Robert- 
