- 
May 9, 1884.] 
persevere,’ is not wholly satisfactory. In one edition 
of the Principia which lies before me, I find the 
statement that ‘every body. . . endeavors to perse- 
vere in its present state,’ etc. Here, certainly, we 
begin to see some trace of Mr. Hall’s ‘ inertia;’ and 
I should not be surprised to meet with the state- 
ment, in full harmony with his views, that ‘every 
body tries to endeavor to persevere,’ etc. 
The beginner in physics is certainly liable to be 
confused in his endeavor to grasp this idea, — the 
idea of the mysterious resistance which Mr. Hall 
illustrates in his string-pulling; but his confusion 
will be vastly increased when he comes to grapple 
with the proposition, that ‘* we must distinguish very 
carefully between inertia itself, a property of matter, 
and the resistance which matter can exert in virtue 
of that property.’’ comparing it, as Mr. Hall does, 
with that property in virtue of which a man can 
exert force, and the force which he may be actually 
exerting at any time; and particularly when he is 
told that the resistance which he has considered is not 
the body’s inertia, but is merely the inanifestation of 
that property! 
The unquestionable tendency of all this is to cause 
the student to attribute to the word ‘inertia’ some 
occult meaning. Most teachers of physics have en- 
countered this condition of things, and have found 
some trouble in ridding their pupils of it. 
Now, a brief analysis of Mr. Hall’s own statements 
will unveil the mystery. If he had tied his string to 
the ghost of a fifty-pound ball, the resistance offered 
would have been nothing; at least, we may so affirm, 
in the present state of our knowledge in regard to 
ghosts. But the string was tied to a mass, and when 
he pulled it, he learned, that, in order to do work, 
work must be done. In short, the word ‘inertia,’ 
when properly used, is synonymous with ‘ mass;’ and 
it is so used by nearly if not quite all the first au- 
thorities. There is, therefore, nothing mysterious 
about it, and, I may add, scarcely any reason for its 
use at all. 
Mr. Hall mentions Maxwell, and Thomson and Tait, 
as apparently sustaining him in his view of the mat- 
ter, quoting to a limited extent from the first. 
Thomson and Tait, in their Natural philosophy, 
although not affirming that matter ‘endeavors to 
persevere,’ etc., do say that ‘‘ matter has an innate 
power of resisting external influences, so that every 
body, as far as it can, remains at rest, or moves uni- 
formly in a straight line.’’ And this innate power is 
called ‘the inertia of matter.’ It is declared to be 
proportional to the quantity of matter in the body, 
and is afterward used as synonymous with mass. 
This assertion of the existence of an ‘ innate power’ 
bears the stamp of high authority, and one ought to 
question it with fear and trembling. But there is no 
evidence, that I have been able to find, that its authors 
believed init themselves; that is, in the sensein which 
many people undoubtedly understand it. I have al- 
ways regarded it as an unfortunate expression, which 
was likely to leave an impression which was never 
intended. 
Professor Rankine, who was not careless in the use 
of terms, uses ‘inertia’ as meaning ‘ mass.’ 
Maxwell is universally admitted to have been a 
man of rare insight into the nature of things; and, as 
he is quoted by Mr. Hall, it may be interesting to see, 
as far as may be, what his position was on the point 
in question. His earliest public expression of opinion, 
as tar as I know, was in his paper, ‘ On the properties 
of matter,’ prepared at the age of seventeen years for 
Sir William Hamilton. This concludes as follows: 
‘and the impossibility of a body changing its state of 
SCIENCE. 
561 
motion or rest without external force is called inertia.”’ 
The next, as far as I know, is found in the Theory 
of heat, quoted by Mr. Hall. But in beginning the 
quotation where he does, Mr. Hall, unintentionally 
no doubt, does Maxwell an injustice. The sentence 
preceding that quoted is a most important and neces- 
sary part of the whole statement [quoted in full by 
C. S. Hastings, above]. 
It will be observed that this gives a perfectly defi- 
nite meaning to the phrase ‘ measurable quantity,’ and 
one quite different from that which might be inferred 
from Mr. Hall’s fragmentary quotation. 
Later came that remarkable ‘ little book on a great 
subject,’ the Matter and motion; and it is a curious 
fact, and worthy of note, that the word ‘inertia’ 
does not occur in this book, not even in its compound 
form of ‘moment of inertia.’ It can hardly be be- 
lieved that this omission was any other than inten- 
tional.- His opinion of the ‘innate power’ may be. 
learned from his review of Thomson and Tait’s 
Natural philosophy [same quotation as given in first 
letter, above]. T. C. MENDENHALL. 
In his article (Science, April 18), Dr. Hall writes as 
follows: ‘‘EHlementary text-books usually speak of 
inertia as a mere inability, —the inability of a body 
to set itself in motion, or to stop itself when in mo- 
tion. Thisis an old use of the term, but certainly 
not the best use.”’ 
Right here, I am constrained to believe, is Dr. 
Hall’s fundamental error or misconception. He mis- 
takes inertia for mass, and, strangely enough, labor- 
ing under this illusion, makes Maxwell use the word 
‘inertia’ where in the text will be found the word 
‘mass.’ For example: Dr. Hall goes on to say that 
‘* Maxwell suggests certain simple experiments which 
the student may perform in order to become ac- 
quainted with that property of matter which he calls 
inertia.’? Now, by reference to the article referred to, 
the reader will find Maxwell’s words to be exactly as 
follows: ‘‘I therefore recommend to the student, that 
he should impress his mind with the idea of mass by 
a few experiments, such as setting in motion a grind- 
stone, or a well-balanced wheel, and thén endeavor- 
ing to stop it,’’ ete. 
Dr. Hall says, ‘‘ We are driven to the conclusion 
that matter possesses a property in virtue of which it 
offers resistance to an agency which is setting it in 
motion.’? If Maxwell regarded inertia as an entity, 
‘a measurable quantity,’ is it not remarkable that he 
did not even once, so far as I am able to find, use 
it in his incomparable work on Matter and motion? 
If, as Dr. Hall is forced to conclude, ‘‘ matter pos- 
sesses a property in virtue of which it offers resist- 
ance,’ why doesit not resist ? Has a mass of matter, 
free to move, ever been known to ‘stand still’ ? 
Certainly not: the whole science of dynamics will be 
overturned when such an instance occurs. The illus- 
tration given by Dr. Hall verifies our position. The 
fact that his heavy weight ‘is left slightly swinging,’ 
shows that a large mass will not resist the slightest 
force. Of course, the velocity generated will depend 
on the time of application. The whole thing is con- 
tained in the equation, v —" 
small, ¢ must be large to make v considerable. Thus, 
in the case cited, there is an attempt to make v con- 
siderable in a short time (¢): therefore # must be 
large; and it is easily made larger than the string can 
bear, when, of course, it breaks. 
In his second illustration, in which ‘a weak thread’ 
If m is large, and f 
