May 9, 1884.] 
on the specimens or not. This is not the arbi- 
trary ruling of any local office, but the written 
decision from headquarters in Washington. 
Such being the case, exchange of specimens 
with foreign countries is practically prohib- 
ited; and this seems all the more absurd, we 
may even say contemptible, when it is known 
that Christmas cards, and several other arti- 
cles not classed in any way as samples, are 
allowed to be sent at sample-rates; further- 
more, that from several foreign countries, 
packages of specimens are allowed to be sent 
to the United States at the cheap rate. Under 
the circumstances, it may, perhaps, be asked 
whether our Canadian friends are not going too 
far in asking that specimens not exceeding in 
weight four pounds, nor exceeding twenty-four 
inches in length by twelve inches in width or 
depth, be sent at the rate of one cent for four 
ounces. To be sure, such an arrangement 
seems to be eminently proper; and all natural- 
ists should unite in bringing the measure be- 
fore the Lisbon convention. In any event, 
the present embargo on scientific exchanges, 
whether caused by the illiberal interpretation 
of the rules of the postal union by our post- 
office, or by any ambiguity in the rules them- 
selves, should be removed. 
LETTERS TO THE EDITOR. 
x*x Correspondents are requested to be as brief as possible. 
The writer’s name is in all cases required as proof of good faith. 
Inertia. 
As Mr. E. H. Hall (Science, vol. iii., No. 63, p. 482) 
referred to Maxwell, Thomson, and Tait, as the au- 
thorities in regard to the use of the word ‘inertia,’ it 
seems to me it would have been well for him to ex- 
plain what Maxwell meant when, in reviewing Thom- 
son and Tait’s Natural philosophy, he said, — 
** Again, at p. 222, the capacity of the student is called upon 
to accept the following statement : — 
*** 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.’ 
** Ts it a fact that ‘ matter’ has any power, cither innate or ac- 
quired, of resisting external influences? Does not every force 
which acts on a body always produce exactly that change in the 
motion of the body by which its value, as a force, is reckoned ? 
Is a cup of tea to be accused of having an innate power of resist- 
ing the sweetening influences of sugar, because it persistently 
refuses to turn sweet unless the sugar is actually put into it?” 
(Nature, vol. xx. p. 214). 
Did Maxwell mean by these questions to deny the 
statement of Thomson and Tait ? 
S. T. MORELAND. 
Lexington, Va., April 21. 
SCIENCE. 
_--_ 
559 
The method of measuring the inertia of a body, 
proposed by Mr. Ilall in No. 63 of Science, p. 483, is 
identical with a mode of measuring the mass of a 
body. Does he consider inertia as identical with 
mass? Ifnot, wherein is thedistinction? Whatever 
be the language describing it, or the ideas concerning 
it, Newton says it ‘‘ differs nothing from the inac- 
tivity of the mass, but in our manner of conceiving 
il”? Here inertia and mass are, by implication at 
least, not identical. Ww. 
April 23. 
The recent article by Mr. Hall on ‘inertia’ is es- 
pecially to be deprecated, because it may lead many 
to regard the ideas relating to it as in some sense 
indefinite. The source of the whole difficulty is that 
the word has been used in two perfectly legitimate 
senses, — one qualitative, and the other quantita- 
tive. In the qualitative sense, it simply implies the 
truth of Newton’s first law of motion: in the quanti- 
tative sense, it is mass, and nothing else. This double 
use of the word has been fully recognized for a gen- 
eration by all accurate scientific thinkers; and, on 
account of this ambiguity, all careful writers and 
teachers have practically long since abandoned it. 
Above all, it ought to appear in no text-book, just 
because it has a double sense. 
This statement as to the usage of careful teachers 
is directly opposed to that of Mr. Hall, who mentions 
Thomson and Tait, and quotes Maxwell in support 
of the position which he occupies. As no teacher is 
clearer in his presentation of elementary ideas, nor 
more precise in his choice of words for conveying 
them, than Maxwell, either my statement or Mr. 
Hall’s quotation demands revision. That the latter 
alternative is the proper one, I shall prove by quoting 
the whole of the passage of which Mr. Hall quotes 
only a portion of one sentence: — 
“In a rude age, before the invention of means for overcoming 
friction, the weight of bodies formed the chief obstacle to setting 
them in motion. It was only after some progress had been made 
in the art of throwing missiles, and in the use of wheel-carriages 
and floating vessels, that men’s minds became practically im- 
pressed with the idea of mass as distinguished from weight. 
Accordingly, while almost all the metaphysicians who discussed 
the qualities of matter assigned a prominent place to weight 
among the primary qualities, few or none of them perceived 
that the sole unalterable property of matter is its mass. At the 
revival of science, this property was expressed by the phrase, 
‘the inertia of matter;’ but while the men of science under- 
stood by this term the tendency of the body to persevere in its 
state of motion (or rest), and considered it a measurable quan- 
tity, those philosophers who were unacquainted with science 
understood inertia in its literal sense as a quality, —mere want 
of activity, or laziness. 
‘* Even to this day, those who are not practically familiar with 
the free motion of large masses, though they all admit the truth 
of dynamical principles, yet feel little repugnance in accepting 
the theory known as Boscovich’s,—that substances are com- 
posed of a system of points, which are mere centres of force, 
attracting or repelling each other. It is probable that many 
qualities of bodies might be explained on this supposition; but 
no arrangement of centres of force, however complicated, could 
account for the fact that a body requires a certain force to pro- 
duce in it a certain change of motion, which fact we express by 
saying that the body has a certain measurable mass. No part 
of this mass can be due to the existence of the supposed centres 
of force. 
_ “TI 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 grindstone, or a well-balanced wheel, and 
then endeavoring to stop it; twirling a long pole, ete., till he 
comes to associate a set of acts and sensations with scientific 
doctrines of dynamics, and he will never afterwards be in any 
danger of loose ideas on these subjects. He should also read 
Faraday’s essay on ‘mental inertia,’ which will impress him 
with the proper metaphorical use of the phrase to express, not 
laziness, but habitude” (Maxwell’s Theory of heat, pp. 85, 86). 
It will be observed that Maxwell, instead of callins 
a certain property of matter inertia, and defining it 
