THE TEACHING- OF MECHANICS BY EXPERIMENT. 
267 
To take an illustration which has been supplied by the lecturer and to apply 
it to the present occasion I would remind you that lie introduced to our notice 
an instrument called the absorption dynamometer. The object of the apparatus 
is to measure the power of a certain machine and if for a moment 1 may apply 
the term ‘machine,’ in no disrespectful sense, to our brilliant expositor—not as 
an idle machine but as one full of brains and instinct with life and energy—I 
think I may look upon ourselves as the absorption dynamometer aud may say 
that our indication is that the machine is of unusual power and that Pro¬ 
fessor Ewing has vastly entertained and informed us (loud applause). 
REPLY. 
Professor Ewing: I am extremely grateful to you for your reception and 
for the kind way in which various speakers have referred to the lecture, I can¬ 
not but feel the subject is one of no great interest to a general audience. 
Professor Dunkerley is quite right in saying that the absence of a laboratory 
saves the teaching staff. When I was a student of engineering my professor, a 
most excellent teacher, came to the university at nine o’clock, and at ten he went 
away again, and had the rest of the day to himself. The modern professor of 
engineering finds the whole day too short to spend in his laboratory. 
I agree that it would be a very bad thing indeed if the engineering 
student were ever brought into such a condition by the use of experi¬ 
ment that he had lost the faculty of reasoning out things with the help of his 
brains and his drawing pencil. I do not think that experiment is likely to have 
that effect, because in reasoning out things what one is constantly doing is to 
revert back consciously or unconsciously to the things one has experienced, and 
the larger the volume of experience that you build up and store in your memory 
the better for the reasoning. 
Professor Greenhill challenged me on the use of the words“mass ’’and “weight,” 
I admit that in the exigencies of a lecture, when one is struggling to express one’s 
idea by any word at all, the wrong word very probably slipped out. But I would 
point to the ballistic pendulum (Pig. 4), as an illustration of what I conceive 
should be the difference of usage between the words “mass” and “weight.” 
When I spoke of determining the moment of inertia of this beam, I said that 
you could do it by observing the period of oscillation as it stands at present and 
then put into the pots two equal “ masses.” It is the mass of these pieces of 
matter that effects the moment inertia, their weight has nothing to do with it. 
They might have no weight at all; so long as they retain the other properties 
which a mass possesses they will still act in causing any increase in the moment of 
inertia. Again, when I said that one way of finding the moment of inertia was 
to put a small “weight ” into one of the pots, I was referring to the characteristic 
of the piece of mafter which is then involved. If you put a small piece of metal 
into one of the pots you cause a statical deflection in consequence of the weight 
of that piece of metal, and submit that the word “ weight ” is appropriate then, 
whereas the word “ mass ” is undoubtedly the more appropriate word to use in the 
former case (applause). 
Mr. Squeers certainly quarrelled with his demonstrator, but it was not due to 
any incompetence on the part of Nickleby. I have never known what it was to 
quarrel with a demonstrator ; it would be an extremely impolitic thing to do and 
so long as one had a demonstrator like Professor Dunkerley it would be quite 
impossible. 
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