ee 
560 
quantitatively in accordance with Mr. Hall’s state- 
ment, is very careful to avoid using the term, putting 
it between quotation-marks in the only place where 
it enters. In short, in so far as a somewhat careful 
inspection of the book from which the above quota- 
tion is made, of his admirable tract on Matter and 
motion, and of his treatise on Magnetism and elec- 
tricity, warrants me, I make the assertion that Max- 
well never uses the word ‘inertia’ in a quantitative 
sense. J am confident that the word does not enter 
into the elementary book on mechanics in any sense. 
In connection with the last paragraph from Max- 
well, I quote a sentence from Mr. Hall’s article (the 
italics are mine): ‘‘ Maxwell suggests certain simple 
experiments which the student may perform in order 
to become thoroughly acquainted with that property 
of matter which he calls inertia.’’ 
Mr. Hall asserts, also, that Thomson and Tait use 
‘inertia’ in the same sense which he recommends. 
As Maxwell’s employment of the term is so different 
from what we should suppose from the article in 
question, I had the curiosity to look into the usage 
of the other authors named. I find the following 
passage, which forms § 216 of Thomson and Tait’s 
Natural philosophy, vol. i., part i., new edition: — 
‘* Matter has an innate power of resisting external 
influences, so that every body, so far as it can, re- 
mains at rest, or moves uniformly in a straight line.’’ 
‘“This, the inerlia of matter, is proportional to the 
quantity of matter in the body; and it follows that 
some cause is requisite to disturb a body’s uniformity 
of motion, or to change its direction from the natural 
rectilinear path.”’ 
This confused definition offers a marked contrast 
to the clear and extended definition of mass contained 
in sections which precede it. It is confused, because 
it admits of a wholly logical but erroneous conclu- 
sion. According to the definition, if we double the 
quantity of matter in a body, we double the inertia 
of the matter present, and thus quadruple the inertia 
of the body. This is absurd. What is meant, but 
not written, is, that the inertia of a body is propor- 
tional to the quantity of matter in the body. Let us 
consider this amended form, and write Zand M for 
inertia and quantity of matter (or mass) respectively: 
then the assertion is, that 
HSN LAG. 
where YX is a function of any thing or every thing 
except mass. Now, experience shows us that J, how- 
ever defined, does not depend upon time, position, 
temperature, electrification, or, in short, upon any 
change in physical condition. We must conclude, 
then, that 
A = C, a constant, and 
EOE 
The numerical value of the constant will, in any 
case, depend upon the system of units selected for 
measuring J and M: therefore we may so select the 
system, that C becomes equal to unity, whence 
oe 
_ Here we see a case where an unnecessary, and, as 
it seems from a casual inspection of the following 
portions of the work, unused term is introduced as a 
survival from the period of ‘the revival of science.’ 
Of course, the passage does no harm to those who 
are competent to read the work which contains it: 
nevertheless, Maxwell would not have used it. 
It is worth noting, that Mr. Hall, in the last para- 
graph of his article, finally gives a definition of mass 
as a quantitative definition of inertia. Of course, 
this is the only quantitative notion which can be 
attached to it. ; 
SCIENCE. 
[Vor. II., No. 66. 
A passage in the article under discussion reads, 
‘‘ Text-books too frequently say, in such a connection, 
that ‘masses of matter receive motion gradually, and 
surrender it gradually,’ or that ‘it requires time to im- 
part motion to a body as a whole,’ — statements from 
which the student is in danger of getting the idea, 
if indeed he gets any idea, that the timeis required in 
order to draw things taut within the body, and get 
its particles to acting upon each other, somewhat as 
it takes time and a succession of jerks to take up the 
slack of a freight-train while it is being sta:ted.”’ 
Unlike its writer, I should recommend the sentences 
within quotation-marks to the special attention of the 
student, and emphasize the fact that time is required 
to transmit motion from one part of a body to an- 
other by the statement, that, in physics, this time is 
known as the measure of the velocity of propagation 
of a wave of disturbance. Finally, if I used the_ 
illustration of the freight-train (not a bad one in its 
way), I should be careful to explain to the student 
that the jerks are due only to the fact that the train 
is not mechanically homogeneous. 
Obviously, the discussion of the term ‘inertia’ is 
not of the slightest scientific importance at this stage 
of scientific development; but it.is of enormous ped- 
agogical importance that loose ideas should not be 
taught. I have been prompted to the above remarks 
by appeals from some, who, supposing they had 
definite notions of elementary mechanics, had been 
led into confusion by Mr. Hall’s statements. 
C. S. HASTINGS. 
Baltimore, April 24. 
In Science, No. 63, Mr. E. H. Hall makes an at- 
tempt to clear away the mistiness which he seems to 
have discovered in the use of the word ‘ inertia.’ No 
word in the English language deserves more sym- 
pathy than this. It has been knocked about so con- 
stantly that it must long ago have given up all idea 
of being able to ‘ persevere in a state of rest.’ Lately 
there have been many indications of an intention to 
put it on the retired list in the near future, and for 
the present to assign it to such duties as it may be 
capable of performing without injury to itself or 
others. But Mr. Hall inconsiderately orders it to the 
front, and insists on endowing it with a real vitality, 
which, in the opinion of the writer, renders it capable 
of doing a good deal of harm. 
Much of the confusion in the use of the word ‘ iner- 
tia’ has originated in the various interpretations of 
Newton’s first law. It is indeed curious to see how 
many different versions of this celebrated statement 
- may be found in a half-hour’s search. 
Thomson and Tait, the restorers of Newton, say, 
‘Every body continues in a state of rest,’ etc. To 
this form of statement it is difficult to object in any 
way. It is a simple statement of a fact, the denial 
of which ‘‘is in contradiction to the only systems of 
doctrine about space and time which the human mind 
has been able to form”? (Clerk Maxwell). This ver- 
sion of the first law is identical with that of Tait in 
his Recent advances. 
But another translator uses the word ‘ perseveres ” 
instead of ‘continues,’ — the rendering so wisely cho- 
sen by Thomson and Tait; for ‘to persevere’ means, 
by common consent, something more than ‘to con- 
tinue.’ Webster says, ‘To persevere is to continue, 
in spite of discouragements,’ etc. In an excellent 
and modern treatise on physics, the law is written, 
‘Every body tends to persevere,’ etc., in which, evi- — 
dently, ‘persevere’ is used in the generic sense of 
‘continue,’ but in the ordinary sense, to ‘tend to 
