1921 .] 
Farr.—Relativity. 
245 
Now, it is fortunate that, as I was able to fall back on a scientific skit-— 
viz., “ Flatland ”—to lead you to understand what vagaries a body moving 
in three dimensions would appear to perform if it were observed by a being 
only conscious of two, and so to show you that the extraordinary shorten¬ 
ings and lengthenings which it became necessary to suppose matter was 
undergoing if we were to explain it on this three-dimensional world of our 
senses could be naturally explained by four dimensions—as I say, it was 
possible to use “ Flatland ” to help me there, so it happens that another 
and better-known novel of our childhood comes to our aid in explaining to 
you Einstein’s idea of gravitation Most of us have surely read A Voyage 
to the Moon , by Jules Verne, and will remember how a capacious cannon¬ 
ball was designed, in which the voyagers lived, whilst the cannon-ball was 
shot out of a special cannon with a speed enough to reach the moon. Now, 
it will be remembered that the dog died and was dropped overboard, and 
continued, to the surprise of the occupants of the projectile, to move along 
with it. It did not, that is to say, drop back to the earth. Mr. Verne says 
that at one point between the moon and the earth the travellers noticed 
that gravity had disappeared ; but in that remark he was wrong, for 
gravity ceased immediately the projectile left the gun, as, indeed, the move¬ 
ments of the dog showed. If one of the inhabitants of the projectile had 
given a jump inside it he would have risen to the top of it and have stopped 
there. His companions might have put it another way and said he had 
fallen on his head on the bottom of it ; but whichever we may call the top 
and the bottom of the projectile, that thing which we are accustomed to 
call the gravitation of the earth had ceased within it. Now, why was this ? 
It was because the projectile, its inhabitants, its furniture, the dog, and all 
that it contained, were, the moment they left the earth’s surface, free to move 
with the acceleration of the particular point at which they happened to be. 
Their velocity was no more constant than the velocity of a stone falling 
off a cliff’s edge to a lower level. At the particular point mentioned 
by Mr. Verne, between the earth and the moon, the velocity was for 
the moment constant. Up to that point, as they left the earth, it had 
been decreasing. It reached a minimum at that point, and thereafter 
began to increase as they approached the moon. But we talk of 
“ increasing ” velocity, and “ decreasing ” velocity, and “ constant ” velo¬ 
city. What do we mean by these expressions ? We are in all this language 
tacitly, and perhaps unconsciously, supposing that their motion has been 
judged from a being on the earth, which we consider to be at rest. They 
were not on the earth, and, had you asked them, they would have said 
nothing about these alleged increases and decreases. They would have said 
that after the unpleasantness of the preliminary bump was over they had a 
very peaceful voyage—that they found an unaccountable lightness about 
themselves, and a remarkable tendency to stand on their heads at the top 
of the projectile ; but they would strenuously deny that they had been 
altering their speeds through space. 
Now, in common, everyday life, when we are looking for the cause of 
a thing or an effect, we look for something that will stop it. When we want 
to stop a motor-car we turn off the petrol-supply, we close the throttle, and 
we say, rightly, that one of the causes of a motor going is that the cylinders 
get a supply of petrol; and many of us can bear most eloquent, though 
perhaps it is profanely eloquent, testimony to the truth of that assertion. 
And so, too, in the more abstruse region of gravitation, Einstein has said 
that any change which an observer perceives in the passing of an event to 
17 —Science. 
