JULY 15, 1915| 
NATURE 
935 

wherever possible results have been found by 
rational rather than empirical methods. The 
design of a steam boiler, so far as strength is 
concerned, is a matter in which the designer has 
but small latitude. In this country he has gener- 
ally to make the results of his calculations con- 
form to the rules of the Board of Trade, or of 
Lloyd’s Committee, or of both. Hence the book 
will be of greater service to the student than to 
the practical designer, although there is much in 
it which will appeal to the latter also. 
Naturally much reference has been made to the 
Massachusetts Boiler Rules, which have been 
used as a model by many States and adopted 
bodily by others. It is of interest to note that, 
with certain exemptions, these rules provide that 
all boilers shall be inspected when installed and 
annually thereafter by the inspection department 
of the district police, under supervision of the 
chief inspector of boilers. In this country the 
periodic inspection of land boilers is left to the 
owner, who generally delegates the matter to 
his insurance company. If the boiler is not 
insured, periodic and competent inspection may, or 
may not, be carried out, and depends entirely on 
the owner’s workmen or engineer. 
The book is well illustrated and contains several 
fully worked out designs. These, and the methods 
of calculation, are of considerable interest and 
will be of service to engineers in this country who 
wish to acquaint themselves with up-to-date 
American practice in boiler design. 
Mechanical Drawing, with Special Reference to 
the Needs of Mining Students. By J. Husband. 
Pp. 79. (London: Edward Arnold, 10915.) 
Price 3s. net. 
Reavers of this excellent text-book will have a 
feeling of satisfaction that a piece of good work, 
evidently much needed, has been conceived and 
carried out in a thoroughly efficient manner. 
Teachers and students connected with mining and 
colliery engineering are much indebted to the 
author for putting at their disposal, in a con- 
venient form, material which will specially appeal 
to them and will prove most helpful in the drawing 
classes and an incentive to study. 
The course is progressive, beginning with some 
excellent advice on the selection and manipulation 
of drawing instruments. The well-executed 
drawings are fully and clearly detailed and dimen- 
sioned, and the descriptions are always suggestive 
in pointing out the leading features of a design, 
and are models of conciseness and lucidity. 
The examples selected are welcome on account 
of their freshness, importance, and_ suitability. 
In order to give an idea of the scope of the work 
we may instance the plates on bolt and rivet 
fastenings; built-up work, such as stanchions, 
girders and pump quadrants; pedestals, shafts 
and axles for wagons and winding engines; mine 
cages and colliery tubs; haulage clips and safety 
hooks; mine pumps; pneumatic hammer drills. 
We strongly recommend teachers of elementary 
machine drawing to consult this most useful 
manual. 
NO. 2385, VOL. 95] 

| let me see. 
LETTERS TO THE EDITOR. 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents, Neither 
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 is 
taken of anonymous communications. | 
The Use of Cotton for the Production of 
Explosives. 
WHILE in no way attempting to dispute the authority 
of the writer of the article on gun-cotton in NaTure 
of July 1, I should like to take the opportunity of 
expressing my surprise at the conclusions there 
reached. On general grounds one would anticipate 
that plant-products, such as cellulose and alcohols, 
could be prepared in any country, so long as plants 
grew; they stand in sharp distinction from metals and 
petrol, which are not of universal occurrence. 
It scarcely seems likely that the suitability of cotton 
as the cellulose basis tor certain explosives can be 
due entirely to chemical peculiarities, for although 
cotton is a typical cellulose it contains many impurities 
—mineral salts, dead protoplasm, cuticle, waxes. 
These cannot all be entirely removed in course of 
manufacture, and it is presumably their presence 
which necessitates the very careful blending employed 
in preparing cotton for nitration. It seems more 
likely that the matter is one of physical properties, the 
thin-walled, hollow, pitted cylinders of the cotton hairs 
offering a large surface to reagents. 
But if this were the prime advantage of cotton, it 
is not one which need inhibit the use of other forms 
of cellulose for the making of ‘‘gun-cotton.” Even 
the best cotton is by no means uniform from hair to 
hair, and the waste cotton used for nitration is still 
more irregular; consequently, it seems likely that an 
artificial cellulose might be made even more uniform 
than cotton. It would scarcely be possible to make 
such a product with the same ratio of surface to mass 
as is found in natural cotton hairs, but this ratio could 
at least be made constant, e.g. by dissolving the cellu- 
lose and squirting thin threads of it, in the form of 
‘artificial silk.’’ The trade in these artificial sills is 
mainly Continental, and has been developing rapidly 
during the last ten years, so that there should be no 
lack of knowledge of the process in Germany. 
Undoubtedly ‘‘ gun-cotton ” made in this way would 
be quite different from true gun-cotton. All values 
would have to be re-computed; the trajectory, with the 
sighting and timing, would be altered, and a great 
deal of extra work would be thrown on all concerned, 
but with all submission to the chemists, to whose 
domain this matter belongs, I venture to think that 
it may be possible to turn out perfectly uniform “ gun- 
cotton” with cellulose derived from plants other than 
cotton. The question of cost is immaterial in the 
circumstances, and although it cannot be doubted that 
the cutting off of Germany’s cotton supplies would 
hinder the German guns, yet it seems likely that a 
fairly effective substitute might be devised by the 
nation of technologists. 
Carlyle may le quoted in this connection. ‘* Inter- 
rupted Commerce and the British Navy shut us out 
from saltpetre; and without saltpetre there is no gun- 
powder. Republican Science again sits meditative. 
. . . What of saltpetre is essential the Republic shall 
not want.” W. LAwRENCE Batts. 
Little Shelford, Cambridge, July 7. 

I HAVE read not only with interest, but also, I hope, 
with some instruction to myself, the admirable letter 
from Mr. Balls which you have been kind enough to 
I am entirely in accord with him in his 
