.- 
THURSDAY, NOVEMBER 19, 1896. 
THE FORCE OF ONE POUND. 
Elements of Mechanics. By Thomas Wallace Wright, 
M.A., Ph.D. Pp. v + 372. (New York: Van Nos- 
trand. London: Spon, 1896.) 
HIS is a good elementary treatise ; not too elemen- 
tary, and yet a book that hard-working students 
may use from the beginning of their studies. It is a pity 
that the author ignores altogether the sort of work that 
is now getting to be very common, even in the older | 
universities—experimental laboratory work in mechanics 
—but we have here an excellent text-book, even for 
students who are following an experimental course. 
The author wisely assumes that some knowledge of 
the calculus may accompany a very elementary acquaint- 
ance with algebra and trigonometry, and he introduces 
calculus symbols freely in places where other authors 
are apt to evade their use, through a wrong notion that 
two pages of algebra and Euclid afford better mental 
training than a line of integration. I think that he is 
right, for every student possesses the fundamental idea 
of the calculus, and knows the use of squared paper 
before he knows algebra, and it is very easy to teach 
him the use of the symbols of differentiation and 
integration. The more advanced subjects include S.H. 
motion, forces in hinged structures, moments of inertia, 
D’Alembert’s principle, the dynamics of rigid bodies, 
strain and impact. The exercises are very numerous ; 
most of them are good, and many of them are of an 
original and taking kind, 
On the other hand, I must say that I could not have 
believed the book to be a new edition of an older work, 
had the author not made the statement. It is in need 
of much correction, and not only of printer’s errors, such 
as errors in decimal points. Then there is a mistake 
calculated to work great harm at page 12, line 6; the 
curve representing velocity of a piston as ordinate and 
space as abscissa, ought not to be like a curve of sines 
or cosines, being really nearly circular. Such mistakes 
as “That this principle (the independence of forces in 
producing accelerations) is not axiomatic, is evident from 
the opinion of Descartes . . . ” (page 49), are, I believe, 
due only to careless writing ; for the author surely does 
not mean that the opinion of Descartes settles the matter. 
There is a different sort of carelessness in (page 62, 
line 17) “At the close of the last century, in different 
parts of the world, the word found was applied to 391 
units of weight, ...” The author means “ 391 different 
kinds of unit... .” As the book is not a metaphysical 
treatise, it seems hardly fair to put before a student the 
exercise, “ Discuss the argument of Zeno of Elea against 
motion” ; “Since an arrow cannot move where it is not, 
and since it cannot move where it is, it therefore follows 
that it cannot move at all.” There is a reference for 
help to vol. iv. page 263 of Carlyle’s “Friedrich” ; but as 
I have only one edition of Carlyle, the reference is not 
of much value. The author is disappointing in his 
parallelogram of forces. Seeing that all metaphysical 
proof has failed, we must frankly adopt the plan of T and 
T’, and say that the parallelogram of forces is included 
No. 1412, VOL. 55] 
WATORE 
49 
in Newton’s laws of motion. It is interesting to note the 
disappearance, from all books, of the older proofs of 
fundamental principles which used to form so large a 
part of the student’s work. They were generally based 
upon the assumption that because we could not conceive 
of something or other, therefore it could not exist ; thus 
giving a premium to the limitation of our faculties. ‘The 
method of reasoning is still much followed, but it is not 
usual to state it so ingenuously ; Maxwell uses the method 
twice in his “Matter and Motion.” In page 80, the 
author takes a line FE F to represent the “force of the wind” 
(whatever that may be, outside the leading article of a 
daily paper), and he says that the component GF of EF, 
perpendicular to the sail, is the effective component in 
propelling the ship. These statements are wrong in such 
various ways that it might be thought they could do no 
harm, but in truth statements could hardly be framed 
to do more harm. The exercise in which a student is 
asked to criticise Hiawatha’s achievements, reads as if 
the author had not made any attempt to understand 
the most transparent of poets; and I am sorry to say 
that some others of the original exercises are really only 
enlivening because they show that, like Silas Wegg, the 
author is fond of “ dropping into” literature. 
If there are these faults, how does ic come that I like 
the book, and recommend it for the use of students ? 
Because it is the work of a man who really thinks about 
the pedagogy of science ; and a man who really thinks, is 
not to be met with every day; he is a good teacher, 
even if there isa mistake in every page The author 
knows his T and T’ well (I imagine), and does not much 
depart from their excellent methods. The book is one 
of the best introductions that I have seen, to the study of 
applied mechanics, and therefore, as an engineer, I like 
it. It has the fault (to me), but in a less degree than all 
the best English treatises, of being what is sometimes 
called “ orthodox” in regard to the “ British unit.” 
1 think that the only act of the late Prof. James 
Thomson which was not altogether excellent, was the 
invention of the word “ poundal.” But he did not invent 
the unit to which the name is given; the inventor of the 
unit has caused it to be true that students are never sure 
in their dynamics calculations. Engineering students 
dare not for their lives speak of foot-poundals of work 
and poundals of force among workmen or foremen ; and 
in what place these quantities are familiarly used or 
needed, except examination rooms, I do not know. To 
support an artificial and unnecessary system, the old and 
excellent term “the force of a pound” is maligned in 
every text-book. It is said to be a variable unit, and 
according to the definitions of its enemies, it is a variable 
unit. As if when I say that a certain force is a pound, | 
mean the gravitational force on a certain piece of metal 
if it were on the moon or at the centre of the earth. 
When an engineer says that the pressure of steam is 
100 pounds per squate inch, there is absolutely no 
vagueness about his statement. For twenty-six years | 
have used as my unit of force the gravitational force at 
London on a certain piece of metal called by law a 
weight of one pound, kept in London, My unit of mass 
is the mass to which unit force gives an acceleration of 
1 foot per second per second, and my students use these 
as engineers’ absolute units. When they are told “a 
D 
