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562 
is ‘pulled gently and steadily,’ is the reason that the 
fifty-pound weight acquires a greater velocity, because 
the weight resists less (if so, then resistance is less 
than itself), or because the time of application is 
greater ? 
In elementary works on physics, the word ‘ inertia’ 
should be seldom used, lest the pupil acquire the im- 
pression that inertia is an entity. Most exact writers, 
foremost among whom is J. Clerk Maxwell, carefully 
avoid the use of the word. But if Dr. Hall’s quasi- 
definition, given in the last paragraph of the article 
under discussion, is to be accepted, then must the 
word necessarily become one of constant use. Itisa 
pity that Maxwell has not given us a definition of ‘an 
inertia unit.’ We shall be pleased to have Dr. Hall 
supply the desideratum. A. P. GAGE. 
In my article on ‘Inertia’ I was mainly concerned 
for the distinct recognition of a physical fact. My in- 
terest in the word ‘inertia’ wassecondary. Professor 
Mendenhall and Mr. Gage appear to deny the reality 
of the ‘resistance’ of which I spoke in defining iner- 
tia. I said, ‘‘ Matter possesses a property in virtue 
of which it offers resistance to an agency which is 
setting it in motion.’? Professor Mendenhall at- 
tempts to avoid the idea of a resistance in explaining 
the fact that force is required to set a body in motion, 
by speaking of the work done. ‘The attempt seems 
to me entirely unsuccessful, unless he has some 
unusual definition of the word ‘work.’ According 
to Maxwell (Theory of heat, 4th ed., p. 87), ‘ work 
is done when resistance is overcome;’ and, though 
he does not say that work is done only when resist- 
ance is overcome, no reader of Maxwell will deny 
that he meant that. This, by the way, is the only 
reply I need make to my critics’ use of Maxwell’s 
tea-and-sugar illustration; for certainly Maxwell con- 
sidered setting a mass in motion to be doing work. 
With this I leave the question of physical fact, and 
come to that of the word or words used to denote 
that property which I have called ‘ inertia.’ 
In using the word ‘inertia’ as I did, I knew per- 
fectly well that I assigned to it a meaning sometimes 
given to the word ‘mass.’ I knew that Maxwell, in 
the very passage of which I quoted a part, and of 
which Dr. Hastings has quoted the whole, used 
‘mass’ as I have used ‘inertia.’ It was my belief, 
however, and it still is, that Maxwell, in that famous 
chapter, used ‘mass’ in two senses. He does use it 
as I have used ‘inertia,’ and in that case defines 
it as a ‘property of matter’ (the italics are mine). 
Elsewhere in the same chapter he says, ‘‘ What is 
really invariable is the quantity of matter in the body, 
or what is called in scientific language the mass of 
the body,”’ etc. (the italics are mine). 
As to Maxwell’s use of the word ‘inertia,’ I was 
in error. I certainly spoke as if he gave undoubted 
sanction to the word in the sense in which I have 
used it. This I had no right to do, for he merely 
states what others have meant by this word. Any 
one, by reading the passage which Dr. Hastings has 
quoted from Maxwell, will see all the excuse I have 
to offer for my blunder. 
Dr. Hastings admits that Thomson and Tait use 
the word ‘inertia’ to denote that property of matter 
for which I have used the same name; but he says 
that their statement is confused. This criticism is 
just; but it is irrelevant, unless Dr. Hastings means 
to imply that Thomson and Tait wrote ‘inertia’ 
where, in a clearer moment, they would have written 
‘mass.’ Moreover, his commendation of their defi- 
nition of the latter word might lead one to infer 
SCIENCE. 
that Thomson and Tait use ‘mass’ as Maxwell does 
in the passage he has quoted. What, then, is their 
definition of ‘mass’? Itreadsthus: ‘* The quantity of - 
matter in a body, or, as we now call it, the mass of a 
body,”’ ete. (art. 208). 
And now what is the practice of my critics in 
the use of the words ‘inertia’ and ‘mass’? In the 
preface of Mr. Gage’s Elements of physics, we read, 
“Dr, C. S. Hastings of Johns Hopkins university has 
read the larger portion in manuscript, and the re- 
mainder in proof-sheets.’’ On p. 8 of this book I 
find, ‘‘ By the mass of a body we understand the 
quantity of matter in it,’ and on p. 20, ** The term 
mass is equivalent to the expression quantity of mat- 
ter.’ Of course, the word ‘mass’ occurs in many 
other passages of the book; but I have discovered no 
case in which it appears to denote any thing but 
quantity of matter. 7 
As to the use of ‘inertia’ in the same book, on p. 
90 I find, ‘‘ This inability is called inertia. Evidently 
the term ought never to be employed to denote a 
hindrance to motion or rest.’’ But when we come 
to the subject of centrifugal force, p. 101, we read, 
“* Centrifugal force has, in reality, no existence: the 
results that are commonly attributed to it are due 
entirely to the tendency of moving bodies to move in 
straight lines in consequence of their inertia.” 
Now, one of these results is the maintenance of the 
solar system. Why do not the planets, obeying the 
law of gravitation, fall into the sun? According to 
the teachings of this book, we must answer, ‘* Simply 
because of their ‘ utter inability’ to put themselves 
in motion, or to stop themselves, although this in- 
ability must never be understood as a ‘ hindrance to 
motion or rest.’’’ <A little farther on in the book we 
read, it is true, that ‘‘ to produce circular motion, the 
centripetal force must be increased... as the mass 
increases.’”’ ‘Mass’ enters here when the book 
speaks of numerical relations; but we see, that, when 
it attempts to explain ‘centripetal force,’ it appeals 
to ‘inertia,’ and says nothing whatever of ‘ mass.’ 
I think it not too much to claim that ‘ mass,’ used 
to denote that property of matter which Thomson 
and Tait call ‘inertia,’ is comparatively rare, while 
one can hardly take up a book upon physics without 
finding ‘mass’ used in the sense of ‘ quantity of 
matter.’ That an exceedingly intimate relation 
exists between inertia as I have defined it, and mass 
as commonly defined, I am well aware. Thomson 
and Tait’s words are, ‘‘ This, the inertia of matter, 
is proportional to the quantity of matter in the body.”’ 
I should prefer to say, bodies of equal inertia (see 
the last paragraph of my article on ‘ Inertia’) are 
assumed to contain equal quantities of matter. 
Quantity of matter, in this sense, is called ‘ mass.’ 
If it seems best to use ‘mass’ to denote also the 
property of matter which Maxwell undoubtedly does 
denote by it, let us so use it; and, by all means, let 
its double meaning be distinctly recognized in the 
elementary text-books. To me it seems far wiser, 
however, to use the two words, ‘ inertia’ and ‘ mass,’ 
substantially as Thomson and Tait use them, and to 
rigorously exclude from the text-books the compar- 
atively useless ‘inability ’ definition of inertia. 
E. H. HALL. 
Silk-culture in the colonies. 
The term ‘silk-balls’ was doubtless employed at 
times to designate cocoons; but that is quite dif- 
ferent from ‘raw-silk’ and ‘ raw-silk balls,’ which, as 
we stated, might more appropriately apply to the 
twisted hanks of raw silk which are so doubled and — 
[Vox. 1IL, No. 66. 
he 
