DECEMBER 24, 1896| 
force. A piece of metal is kept in London; it is defined as having 
there the weight of 1 pound, but only if weighed in a vacuum. 
Because it has this weight it is called 1 pound of stuff. Because 
it has this weight its inertia is said to be that possessed by a 
body whose weight is 1 pound. Now there are your standards 
in one piece of metal and in its environment, and in your instru- 
ments. You cankeep the name 1 pound as the wezg/t according 
to law, or you can call t pound the zzer¢7a of the body, but you 
cannot call it 1 pound of quantity of any material except of that 
particular kind of P#? Whichis more convenient? To call it the 
unit of a kind of P¢ is too restrictive. Tosay that it has 1 pound 
of inertia is not more scientific than saying that its inertia is 
1/3219. As to convenience, note that we almost never need to 
speak of the zzertéa of a body except on our way to a dynamical 
calculation. The claim of wezgh¢ of 1 pound as a unit of force 
is ever so much greater ; for (1) there is the legal unit in exist- 
ence. It might have been defined as the force required to 
elongate a certain spring, but this also would have been indirect. 
(2) All practical men use the unit already, and it is hopeless to 
try to alter their systems of working. All applied mechanics 
and engineeering books written in the English language, from 
Rankine’s superb treatises to the commonest pocket-book, use 
this unit ; not one such book of repute can be mentioned in 
which the Poundal is used and in which this unit is not used ; 
Joule used it, and many others of our best experimenters ; even 
physicists never compare their forces in any other way than with 
the weights of bodies, and the pound is legally the English unit 
of weight. (3) Theunit is so exceedingly easy to understand that 
years have now to be spent in driving it out of a pupil’s mind; 
there is no part of our universe which we can reach where we 
might make experiments, where the unit is not easily derivable 
by a small and often negligible correction from the weight of a 
body whose weight in London is equal to that of the standard 
piece of platinum, and such bodies are to be found wherever 
men buy and sell by weight. 
I think that Mr. Jackson is well answered in a quotation, 
slightly altered, from Dr. Lodge :— 
“To identify quantity of stuff and inertia is barbarous, to 
denote their units by the same name is unwise.” 
By the bye, I should like Dr. Lodge to point out where I 
have lost sight of the dimensions of g and treated it as a mere 
equivalent for 32°18. One might say that it was stupid, or 
thoughtless, or academic, but I hardly think that Dr. Lodge is 
happy in his use of the term ‘‘ illiterate.” Anyhow, I did not 
do it. -Engineers do not do things of that kind. 
Mr. Cumming thinks that a system of theoretical dynamics 
can be built upon our basis ; he does not seem to be aware that 
the system is already built ; the ordinary dynamical expressions 
have no more to be altered when our units are employed than 
-when the C.G.S. units are employed, and we do not need even to 
introduce Dr. Lodge’s method of writing, which gives so much 
trouble in all but the very simplest algebraic expressions, but 
which certainly enables him to use any units whatever. I am 
afraid that it is known only to a few people as yet, but it is 
well worth knowing, being almost a necessity if one uses the 
Poundal or the Hogshead. It is simply this: every quantity 
goes about with a label as of a sandwich-board round its neck. 
Parenthetically I may say that Mr. Cumming is mistaken in 
thinking that engineers use the C.G.S. units in working with 
such fundamental equations as 
+ VES ad 
V=(R+LE 
They use volts, amperes, ohms, secohms, farads, and seconds. 
Why else were the practical units invented? They were 
invented by practical engineers who at the same time invented 
the C.G S. system, because they found that for the working of 
engineering problems, accurate answers and not merely academic 
logic was wanted. Unfortunately the academic 4m trouble was 
introduced, and till it is removed the practical man feels that 
life isa burden. These engineers were, as so many of our best 
engineers have been, trained at Cambridge, and this is one 
reason why I do not like to hear Dr. Lodge throw so much 
blame on Cambridge text-books. My opponents are not in 
agreement among themselves. I wish they were all as catholic 
in their sympathies as he. He wonders why people object, and 
who they are who object to the term centrifugal force. 1 think 
he must know many; anyhow there are certainly some, for 
they have told me so themselves during the last few weeks. 
And even he is now disposed to prohibit the use of a word, 
NO. 1417, VOL. 55] 
a) 
(k= 
1 he 
NARORE 
‘between engineers and teachers of mechanics ? 
177 
never used wrongly by engineers, because the present generation 
of academic persons have found out that their predecessors had 
very wrong notions. 
I do not see why he should speak so disrespectfully about 
that most wonderful property of a body, its attraction by and 
for the earth, which I will call its we¢ght. He calls it ‘‘a 
curious and ill-understood deportment,” and his dislike for it 
is due to its having ‘‘ laid such hold of the engineer's imagina- 
tion that he has begun to think it the most fundamental property 
of matter.” I am glad that he concedes the engineer an 
imagination in spite of his not understanding the so easily 
understood ‘‘ etherial stress.” For my part, I acknowledge 
great ignorance about it, and I think it the most fundamental 
property of matter. 
His students are very happy; they know all about etherial 
stress ; they know how to use all units, any units, to get along 
with no units, and, in fact, Dr. Lodge seems to think that 
what he himself knows, after all his years of study, must also 
be known to his students because he has told them. I may say 
that I also delight in using all sorts of units. 
I perhaps go further than Dr. Lodge, for I regard algebra 
as the best of all mediums for translating phenomena into 
mental processes, and I should not object to the multiplication 
of a cows + 6 bridges with c cows + 5 tons of sugar, if I could 
see any use in giving a meaning to such things as cow x cow 
or cow® (curiously enough, Dr. Lodge would object to cow 
squared as a colloquial reading of cow x cow), or cow x bridge. 
But surely the student described by Dr. Lodge is a very ex- 
ceptional philosopher, a boy of eighteen, with all Dr. Lodge’s 
knowledge. Is he not assuming that because a student gets 
marks on an examination paper, he really knows his subject ? 
How easy it is for a student to get full marks for ‘* What is 
Ohm’s law”? But how many years of his life must elapse 
before he really knows Ohm’s law? Or ‘ What are Newton’s 
laws of motion”? How very easy, and yet how exceedingly 
difficult. Take force = mass x acceleration. A student thinks 
he understands perfectly what you tell him, and can work all 
sorts of exercises on this statement. But let the thing come 
before him in a new form, and where is his knowledge ? 
Dr. Lodge ¢hzzks in all his units, and in my opinion the 
students of whom he speaks cannot think in them. I want to 
put students in the way to the mental position which Dr. Lodge 
postulates as the best. I want them to get into a higher position 
still, that of the engineer who is able even to think of the same 
sorts of terms in one equation being in quite different units. 
Dr. Lodge would dilate on the ignorance shown by this 
engineer. Mr. Barrie, in describing two boys in one of his 
books, says, ‘* Shovel knew everything, but Tommy knew other 
things.” 
In my article I refer to the persistent scorn of the academic 
philosopher for the engineer and the harm that it has done. 
Surely Dr. Lodge might restrain it a little in a public discussion. 
All English-speaking engineers use the force of one pound as 
their unit, and Dr. Lodge sneers at it as the colloquial unit of 
the shire in which an engineer happens to live. The shire is a 
very extensive one. It needs a globe to show it all. Hesneers 
at the Heaven-born engineer of whom I spoke, and of his wish 
to advance in his profession, and the mutilated fragment of science 
and pocket-book information which serves for commercial pur- 
poses. He seems to be very proud of his ignorance of these 
commercial purposes to which so many students of higher 
physics mean to devote themselves, and yet he is not backward 
in expressing his opinions concerning them. He says: ‘* May 
I tell Prof. Perry what is at the root of the perennial debate 
It is the subject 
of acceleration. An engineer’s bodies are nearly always either 
at rest or in uniform motion, their accelerative stages he is 
usually able to ignore.” If anybody can speak on this subject, 
surely it is I. Iwas trained in the shops and at college as an 
engineer, and I have done a good deal of engineering work, 
and I teach mechanics. I beg to say that Dr. Lodge is quite 
wrong in this. When I wrote about professors and engineering 
students I did not once think of him I think now that he has 
written in haste, and that he cannot seriously put forward the 
view that the very most elementary idea of kinetics is unneces- 
sary to the actual professional work of the engineer! Is it, 
then, of no use in any practical work of anybody? Has Dr. 
Lodge no students who think of the forces acting between the 
parts of reciprocating machinery, of the balancing of engines, of 
the action of governors, of the effects of centrifugal force? It 
