226 
iene URE 
[FEBRUARY 17, 1923 

In Definition I, density is taken as the primary 
property of matter, although left undefined by Newton ; 
while Quantitas Materiae, our W or P, is the product of 
density and volume. 
The Materiae Vis Insita of Definition ITI is described 
as the same as Inertia Massae. This, however, is not 
the definition of Massa, but Inertia, although the two 
are treated as the same thing in modern interpretation. 
Newton is not consistent with himself, as asserted, 
in always using Pondus as meaning the attraction 
of the earth on a body on the surface. As often as 
not he uses Pondus in the popular acceptation, as 
in the Act of Parliament,and a search in the “ Principia” 
will reveal numerous instances. 
This distinction, insisted on so carefully in modern 
instruction, was ignored in language and thought till 
about fifty or sixty years ago, when Absolute Measure 
was first introduced into dynamical teaching. 
The artless definition of Mass, as the quantity of 
matter in the body, is near enough to serve in a 
dictionary, as a synonym in one line. It is merely 
the selection of a new name as a label in the long list 
already in Def. I. But a real definition will give at the 
same time the best way to measure the quantity. Ina 
recent Royal Society memoir, on “mass determina- 
tion” as the author is careful to call it, the question of 
its measurement turned on a “ Study of the Balance, in 
its greatest precision, in a projective series of weighings 
of small masses,” the most accurate of all physical 
operations we know. 
Libra, sign in the Zodiac of the Balance, 
appropriate emblem of justice holding the scales. 
It is contrary to the strict legal language of the 
Act of Parliament on weights and measures to start 
off with another artless definition—the weight of a 
body is the force with which it is attracted by the 
earth. At that rate, what is the weight of the moon, 
the sun? The definition is not supposed to apply to 
a body, so long as it is not terrestrial. 
The attraction of the earth on the pound weight 
as the unit of force (gravitation) will never be abandoned 
by the engineer, as it is susceptible to the same degree 
of accuracy of measurement as the operation of 
weighing. 
is an 

But when Tait took in hand the reform of dynamical 
teaching, he altered the equations in our form of 
(r), (2), (3), (4) in a new way, with the view of exter- 
minating g. He discarded the old sui generis mass, 
with unit of g lb., and taking mass in its new meaning 
of the invariable "quantity of matter in the body, he 
measured it in terms of the Act of Parliament unit — 
of weight, the pound weight. This involved him in 
a change in the unit of force, to what was called a 
poundal, such that the engineer’s gravitation unit 
of force, the pound (force), was equivalent to g 
poundals. 
Tait’s change merely amounted to labelling MW 
the quantity formerly labelled W. But he insisted 
on retaining W=Mg, and so measuring what he called 
weight in poundals, contrary to the strict law of the 
Act, and rendering himself lable to a fine for every 
offence. Better if Tait had retained the letter W 
for lb., writing the equation P=Wf, and rejecting 
the useless W = Mg, as perpetuating the old sui generis, 
and breaking the law in the Act of Parliament. 
This trouble of mere terminology would be exorcised 
if the habit was inculcated of always stating the unit 
of a dynamical quantity, as, for example, of a mass 
M, g,a weight W, lb. The engineer refuses to accept 
the poundal or to give a weight W in poundals. Scrap 
the name as useless, except for passing certain examina- 
tions. 
To the masses in general the word mass implies 
a combination of bulk and density as in Definition I, 
as when we speak of mass of stuff, the mass of the 
earth— Die Erde und ihre eigene ungeheure Last ” 
(Mach). In ordinary language the mass will mean the 
multitude, or majority, as in the statement attributed to 
Herbert Spencer, ‘‘ The mass of woman is insensible to 
gravity,” which might mean a reminiscence of the 
ballroom floor ; but this was before the women began 
to take themselves so seriously ; and when we read 
the critic’s snarl of the “ Vast Mass of his writings 
consigned to Oblivion,’ Vast Mass here is forcible- 
feeble for Major Pars. 
The word is spelt Maas in German ; “ Mass fiir Mass 
is the title of the German version of Shakespeare’s 
play ‘‘ Measure for Measure.” 
”? 
Wegener's Hypothesis of Continental Drift.* 
By Puiuir Lake. 
\V EGENER’S hypothesis is based on the idea 
that the continental masses are patches of 
lighter rock floating and moving in a layer of denser 
rock, and this denser rock forms the floor of the oceans. 
Following, with a slight alteration, the terminology of 
Suess he calls the lighter material the Sial and the denser 
layer the Sima. Suess uses the words Sal and Sima, 
and thinks that the Sal covers the globe completely. 
I shall not here discuss the possibility of Wegener’s 
conception. He does not profess to explain completely 
why the continents should move, but he claims to have 
proved conclusively that such movement has taken 
place. It is the evidence on which he relies, and more 
particularly the geological evidence, that I propose to 
examine. 
1 Abridged from an address to the Royal Geographical Society on 
January 22. 
NO. 2781, VOL. 111 | 

One of the arguments on which he lays great stress 
is derived from the relative frequency of different 
heights and depths upon the earth. His diagram of 
frequencies shows two well-marked maxima, one at 
about 1oo metres above sea-level and the other about 
4700 metres below it. Wegener concludes that two 
distinct surfaces standing at these two altitudes must 
have been involved in the subsequent movements. He 
assumes that these surfaces were originally level—or, 
more strictly, equipotential—and that they were the 
surfaces of the Sal and ‘the Sima respectively. He 
holds that if originally there were only one such level, 
the deformation of that level could not produce two 
maxima and “ the frequency | must be regulated accord- 
ing to Gauss’s law of errors.’ 
Tn reality, if it is only a single level that has been 
deformed, it is improbable that the resulting altitudes 
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