432 
roll on a level, though then neither descend- 
ing nor ascending? “Is it because that which 
is heavy is difficult to move not only vertically, 
but also horizontally?” Is it because things 
require an effort not only for lifting them, 
but also for setting them in motion? 
He is evidently groping; his difficulty is, 
that he fails to analyze the meaning of the 
phrase, easy to move, or, to move easily. A 
thing is easy to move when with a slight effort 
in a short time we change its speed consider- 
ably, or give it a large acceleration; it is hard 
to move when we must exert a considerable 
effort to produce the same total change in 
speed, or a slighter effort for a longer time, 
then giving it only a small acceleration. He 
has not distinguished clearly the elements 
inertia, of which mass is the quantitative 
measure, and acceleration, or rate of change 
of speed; but he does see that there is a funda- 
mental distinction between accelerating a body 
in the horizontal direction, the direction in 
which its motion is unaffected by weight, and 
accelerating it in the vertical direction, when 
the weight hindrance is superposed on and 
confused with the difficulty of acceleration. 
His observations being merely sensuous and 
qualitative, he could not know by measure- 
ment the constancy of gravitational accelera- 
tion, nor could he escape from the confusion of 
weight and mass which this same constancy 
made inevitable. 
Again, watching the woodman and his axe, 
Aristotle knew that effort exerted as steady 
pressure would not cleave the block, though 
when swung through the air the blade split 
the wood instantly. It seemed odd to him 
that a continuing effort without motion should 
be unable to achieve an end so easily reached 
by an effort no greater used to produce motion 
of the axe. 
Is it because ... that which is heavy gets more 
movement from its weight when it is in motion 
than when it is at rest? 
For movement read here momentum, or use 
Newton’s phrase, quantity of motion, and read 
mass for weight, and the query sounds much 
more modern, though not yet quite clear. He 
SCIENCE 
[N. 8. Vou. XLVIII. No. 1244 
Was again groping after distinctions later 
grasped by Newton. In his observation there 
was, of course, nothing new; upon such the 
arrow-maker’s craft was built millennia before 
Aristotle; but he was perhaps the first to record 
a feeling of perplexity concerning them. 
In another place (Ch. 81) Aristotle asks: 
Why is it that a body which is already in mo- 
tion is easier to move than one which is at rest? 
This evidently deals with running and start- 
ing friction. His answer is very unsatisfac- 
tory; but this query, as do one or two others, 
touches on the nature of friction, that other 
obstacle which for ages prevented the develop- 
ment of a science of mechanics. 
I have no doubt that these two facts in 
terrestrial experience, the universality of fric- 
tion and the practical constancy of gravita- 
tional acceleration, were not only the chief 
causes of the slow development of mechanics, 
but are also to-day among the main reasons 
why so many, perhaps most, of our athletically 
and mechanically wise students not only fail 
to comprehend the laws of motion—they 
simply do not believe they are so. They are 
confirmed Animists; to imagine a force act- 
ing, they must imagine themselves acting right 
there, or the agent feeling as they would in 
its place. They do not believe in a natural 
tendency of things to keep on going, and ac- 
celeration is to some of them a concept of in- 
accessible refinement. “To describe the mo- 
tions which occur in nature, in the simplest 
way,” or otherwise than in terms of their own 
effort sensations is a problem of small inter- 
est. And with the phenomena thus sensuously 
described, to them a body is hard to throw be- 
cause it is heavy. From qualitative experi- 
ences, such as their predecessors since and be- 
fore the Stagyrite have had in fulness, they 
will never gain conviction of the truth. Ex- 
periments must be devised and set before them 
to induce in them the perplexity Aristotle felt 
in presence of the loaded balance and the roll- 
ing cylinder; for this feeling of perplexity, 
and a revolt against it, are the beginning of 
science; then they must resolve this perplexity 
by experiments and study which will lead them 
