FEBRUARY 15, 1917] 
<8) 6c) aS eh Rais ee 
(3) It was in 1882 that Drzewiecki first pro- 
pounded a theory of propeller action based on the 
. Supposition that if the length of a screw-blade be 
split up into small elements the fluid motion in 
_ the neighbourhood of any element may be re- 
garded as two-dimensional, and independent of 
the differences of shape of the neighbouring ele- 
ments, so that the effect is the same as if the 
element in question formed a section of a cylin- 
der moving through the same medium with 
uniform velocity.. Reference to the same method 
will be found in Lanchester’s ‘ Aerodynamics.” 
But at the time of Drzewiecki’s first papers the 
Principal screws occurring in engineering practice 
were the propellers of ships, and these are some- 
times broad enough to account for considerable. 
divergences between theory and experiment. 
Parenthetically, too, it will be seen from Sir G. 
Greenhill’s “Dynamics of Mechanical Flight ” 
that Newton’s “sine squared” law of resistance 
still figures largely in some of the well-known 
theories of propeller action. 
_ The assumption under consideration is the same 
as that made by the present writer in connection 
with lateral stability under the title, “Further 
Hypotheses regarding Narrow Planes.” Mr. 
Riach’s figures of the blades of modern air screws 
Suggest that these are sufficiently narrow to justify 
an investigation of their efficiency based on this 
admittedly rough-and-ready, approximate method. 
It is the object of the present work to develop 
this method of investigation to its ultimate logical 
conclusions. This is a perfectly definite piece of 
purely mathematical work involving nothing 
harder than writing down integrals, except that 
the solid geometry of the screw and the different 
kinds of pitch associated with it is rather 
puzzling. It would thus be easy for anyone to 
work out all the results for himself, and _ it 
is doubtful whether it is worth paying ten 
shillings to have this done. But even if 
nothing else could be said in favour of the book, 
it is a great gain to have the formule ready 
worked out and displayed in print. And, after 
all, there is a great deal to be said on the sub- 
ject, and there are many minor details which not 
. everyone would think of working out. The vari- 
ous forms of blade proposed and described as the 
“efficiency curve,” “rational,” ‘ normal,” and 
“constructional limit ” outlines are interesting. 
Mr. Riach is under no misapprehensions as 
to what his investigation does and does not 
prove. If the fundamental assumptions are ad- 
mitted, the rest follows as a logical consequence. 
Divergences between theoretical and experimental 
results will depend on the extent to which the 
actual stream-lines of the medium differ from those 
based on the two-dimensional or aerofoil hypo- 
thesis.. The author finds that in many cases a 
fairly close agreement has been found between 
theory and practice, but, as he freely admits, 
there are cases in which the differences may be 
considerable. It is important to bear these facts in 
mind, because there is a certain type of individual, 
unfortunately very common, who cannot, or will 
mot, appreciate the value of an investigation con- 
- NO. 2468, VoL. 98] 
NATURE 
467 
ducted in this spirit, and we can only hope that 
Mr. Riach will not find himself dragged into a 
hornet’s nest of controversy by cranks and fad- 
dists. There is nothing that retards progress 
and wastes time so much as the irrelevant “dis- 
cussions ” which are so often started on a per- 
fectly straightforward piece of work. What is 
now required is further comparison of theory with 
experiment. 
The book also contains chapters dealing with 
the stresses in propeller-blades, their design and 
construction, and other similar matters. It would 
be well, however, to point out that in working 
with a theory which is at best an approximation, 
it would be sheer waste of time to worry about 
evaluating complicated integrals, as a process of 
summation over a finite number of elements would 
be sufficiently accurate. At the same time, many 
of the integrals are comparatively simple, and 
some of them can be simplified by a trigonometri- 
cal transformation to an angular co-ordinate the 
geometrical meaning of which is obvious from the 
figures. 
(4) Mr. Lanchester’s books are too widely known 
to require further comment, and Commandant 
Benoit has adhered fairly rigorously to the original 
text in his translation, not even venturing to 
insert a preface of his own in addition to that 
dated May, 1908, by the author. Probably this 
was the best plan, for everything has now 
changed so greatly that it would be impossible te 
bring a book up to date nearly nine years after 
publication. G. H. Bryan. 
MILK AND THE PUBLIC HEALTH. 
Milk and its Hygienic Relations. By Dr. Janet E. 
Lane-Claypon. Pp. vili+348. (London: Long- 
mans, Green and Co., 1916.) Price 7s. 6d. net. 
HIS book is published under the auspices 
of the Medical Research Committee, which 
is charged with the administration of the Research 
Fund which has become available under the provi- 
sions of the National Insurance Act for the 
advancement of medical knowledge by research. 
Milk has very important relations to the public 
health, but the study of milk has been conducted 
by workers in chemistry, physiology, bacteriology, 
agricultural science, and clinical medicine, and 
the results of their researches have appeared in 
journals devoted to all these branches of science. 
With the view of collating this mass of literature, 
the Medical Research Committee invited Dr. Janet 
Lane-Claypon to assist them in collecting the 
available scientific evidence upon the hygienic 
relations of milk from all the best sources of 
information, however widely scattered, and the 
present volume is the outcome of her labours. 
Not only has a large mass of literature been 
abstracted, but critical summaries are provided 
in addition, and extensive bibliographies are ap- 
pended to the subjects dealt with which will be 
very valuable to future workers. 
The general composition of milk and its organic 
and inorganic constituents are summarised in 
chaps. ii., iii., and iv. The “biological” proper- 
