NATURE 
[Marcu 4, 1915 

metrical system is developed to a considerable 
extent, and is left at a point from which it may 
be easily carried in its correspondence with the 
whole range of ordinary geometrical detail. 
The investigation is made in connection with 
the phenomena of optics. This is, of course, 
necessary in any attempt to analyse the founda- 
tions of any space and time theory. The view 
taken is that the axioms of geometry are mainly 
the formal expression of certain optical facts, and 
the author rightly points out the defects in systems 
some of the axioms of which have this significance, 
while others depend on such things as the pro- 
perties of purely ideal rigid bodies. The signi- 
ficance of all axioms in a logical scheme must be 
of the same character, and the optical character 
is the only legitimate one we have at our dis- 
posal. 
It is not possible in a review to give any account 
of the detailed working out of these ideas. It can 
only be said that their development is extremely 
elegant, and is worthy of being taken as a model 
for any type of geometrical work. The book is 
in the standard form of the Cambridge University 
Press, and bearing in mind the traditional excel- 
lence of this series, it is perhaps superfluous to 
say that the tradition is well maintained in the 
present case. 

APPLIED MECHANICS: AND AMERICAN 
TIMBERS. 
The Mechanical Properties of Wood. By Prof. 
S. J. Record. Pp. vi+165. (New York: J. 
Wiley and Sons, Inc.; London: Chapman and 
Hall, Ltd., 1914.) Price 7s. 6d. net. 
O those unfamiliar with the composition and 
construction of wood it comes as a matter 
of surprise that the “hardness,” calorific value, 
also, to some extent, the strength and certain 
other mechanical qualities, are proportional to the 
apparent Specific gravity. Yet the unexpected- 
ness of these relations vanishes when it is realised 
that a piece of completely dried wood contains, 
in addition to air and insignificant amounts of 
various substances, wood-substance the specific 
gravity of which is approximately (or truly?) the 
same in all kinds of woods. Two pieces of dry 
wood of the same volume thus differ in weight 
nearly solely because one contains more wood- 
substance than the other. Wood is not a material, 
but is a heterogeneous and varied structure, and 
its particular mechanical properties are dependent 
not merely on the amount of wood-substance con- 
tained in its unit of volume, but also on the 
manner in which that substance is excavated in the 
form of strong fibres, weak vessels, and so forth. 
The arrangement, form, and numbers of these | 
NO. 2366, VOL. 95; 

constituents vary widely in different kinds of 
timbers, with the result that these display corre- 
sponding differences in their mechanical proper- 
ties. 
In view of Prof. Record’s botanical work on the 
structure of the woods of the United States, it 
might have been anticipated that the book under 
review would deal considerably with the interest- 
ing, and incompletely investigated, problems con- 
cerning the correlation between the structure and 
properties of timbers. Very different, however, 
are the scope and contents of the book, the sub- 
ject matter of which includes, in order, a brief 
account of the elementary mechanics of materials, 
a special consideration of the mechanical proper- 
ties of wood and of the factors influencing them, 
and a description of the methods of wood-testing 
officially adopted in the United States. It gives 
concise information as to facts gleaned particu- 
larly in that country from experiments conducted 
along the lines laid down by workers in Austria, 
Germany, and Switzerland; and the information 
is illustrated by well-chosen figures, including 
photographs. 
The book, however, is akin to a collection of 
lecture notes dealing with actual experimental 
results obtained, rather than to a reasoned exposi- 
tion of the subject. The resultant curtailment is 
consequently apt at times to lead the student into 
lack of comprehension or to misconception. The 
latter is the case when two definitions of hardness 
are given, the second being “resistance to 
abrasion or scratching.” The truth is that when 
measured by abrasion the resultant estimates of 
hardness agree with those obtained by indenta- 
tion tests and everyday experience with a saw; 
whereas when tested by scratching all woods are 
approximately of the same degree of hardness, and 
that degree is a low one (about equal to musco- 
vite). One feature that takes from the value of 
the book to English students is that in it, as un- 
fortunately in an increasing number of American 
scientific books, the language, terminology, and 
nomenclature are partly foreign to England. Not 
only are the timbers mentioned exclusively under 
American names, but popular technical terms un- 
| known in this country are employed, and scientific 
definitions do not at times accord with those in 
use here. For instance, a unit stress is defined as 
“the stress on a unit of sectional area.” (Not 
thus explicable is the erroneous statement on the 
same page that a “stress-strain diagram” is “a 
diagram or curve plotted with the increments of 
load or stress as ordinates, and the increments of 
strain as abscissee’’; the curve in question sup- 
plied shows that the words ‘increments of ” 
should be omitted.) 
‘ 
