428 
out the use of this name could only with diffi- 
culty be expressed in words.” 
Believing that the history of a growing con- 
cept is sometimes an aid towards a teacher’s 
understanding of his students’ difficulties. I 
have been interested in forming a conjectural 
reconstruction of the history of this force con- 
cept, from the hidden days of pure animatism 
to the time when a distinct separation between 
matter and force concepts began to show itself; 
helping myself meanwhile with such facts as 
archeology scantily shows us about the most 
ancient tools, contrivances and ways of life. 
To put the problem as a question: When and 
how was it learned that very different objects 
may have the same weight? That the same 
object may have different weights? To treat 
of force mainly in the weight form is no 
wrong, on account of the universality of gravi- 
tation and the fact that forces even to-day are 
measured mainly in terms of weight. 
The second question is quickly answered; 
before Richer in 1678 returned to Paris from 
Cayenne with a report on the going of his 
clock in the two places, no one had suspected a 
variability of weight; Huygens concluded from 
this report that bodies in high latitudes fall 
faster, and are heavier, than in low; but even 
now this conclusion remains a deduction from 
refined instrumental observation; no mason’s 
assistant can say from his personal experience 
that it is harder to lift a hod of bricks in Edin- 
burgh than in Quito. To us all, the weight of 
a thing is constant. 
That different objects may have the same 
weight is an extremely ancient idea, so fa- 
miliar as to be a truism, I dare say, even to 
the pyramid builders and their forefathers. 
But I suppose that even truisms were once 
discoveries; this one, perhaps, became the 
property of man because he labored. 
Assuming that sensations of effort are real, 
I would classify them, perhaps naively, in 
three groups: Sensations of 
(a) Effort proper—central, which go with 
the sending of the nerve message from the 
central nervous system. 
(6) Stress—the nerve message reaches the 
SCIENCE 
[N. 8. Vou. XLVIII. No. 1244 
muscles, they contract, their changes in form 
affect the sensory endings of themselves and 
of neighboring parts, as the skin, the joints, 
ete., by virtue of arterial and venous com- 
pression, the circulation and the breathing 
also are affected, and, through them, distant 
parts of the body. 
(c) Yielding—changes in stress occur when 
external bodies give way to the muscles and 
bones, or when the body itself is moved, as in 
jumping. 
Normally (a) and (6) go together; in night- 
mare most of us have felt the will paralyzed, 
the body, apparently not responding to the 
’ centers; in paralysis the separation may be 
permanent; I have read that the separation 
occurs in curare poisoning, when the motor 
nerves no longer actuate the muscles, but con- 
sciousness and sensation remain. 
But (c) varies with the object dealt with, 
and also, for the same object, with bodily 
health and tone. It varies with the way the 
object is dealt with. Its variations, combined 
with the evidence of other sensations, enable 
us to distinguish between the self and the not- 
self, and between the parts of the not-self, of 
the external world. With (a) and (6) but 
without (c) we would know little of the 
mechanical qualities of bodies. With (c).we 
get notions of bodies differing not only in 
color, odor, ete., but also in weight; for to 
move objects, whether to lift, carry or throw 
them, requires effort, and the efforts for lift- 
ing, carrying and throwing a given body are 
of the same order of magnitude. Like bodies 
of about equal extent require like efforts; like 
bodies of unequal extent require unlike efforts; 
but equal extent does not condition equal 
efforts; e. g., a block of wood and a boulder. 
So we can add to the differing qualities of 
bodies given by sensations of color, odor, ete., 
weight and specific heaviness. This effort- 
demanding quality, varying among bodies and 
with the condition of the person, would early 
be abstracted, and the concept weight would 
appear, in positive (heavy) and comparative 
(heavier, lighter) degrees. Weight was found 
to be a quality of solids and liquids univer- 
sally; the sensations of effort hayé not yet 
