244 The N.Z. Journal of Science and Technology. [Jan. 
as it were, realizing that a complete explanation of the universe may be 
for ever outside his grasp. 
And here it is that we come to Einstein’s most brilliant work ; but the 
understanding of it is an exceedingly difficult matter. His theory of gravi¬ 
tation, which alone, so far, has been able to give us any direct check upon 
these relativity ideas, is the outcome of most abstruse and lengthy mathe¬ 
matical analysis. Whilst I can claim with perfect honesty to have made 
an attempt to understand what has been written on the subject, I must 
admit with the same candour that I am sure I have not done so ; but yet 
I may perhaps be able to give you the ideas underlying the theory -— a 
theory which is perhaps the most, revolutionary that has ever been enunci¬ 
ated. Wading through a mass of mathematics, one emerges, after struggles 
with co-variant, and contra-variant, and mixed tensors of the third and 
higher rank, with somewhat nebulous and confused ideas of what it is all 
about, and more confused ideas of how it is ever to be made understandable 
to the general public. We all know what Newton’s law of gravitation is : 
it is that every particle of matter in the universe attracts every other 
particle of matter in the universe with a force which is proportional to 
the product of the two masses involved, and inversely proportional to the 
square of the distance between them. Resulting from this law we find 
that 2 grammes placed a distance of 1 centimetre apart attract one another 
with a force which is 6-6 X 10 of the weight of a gramme. With ordinary 
masses such as we see about us— e.g., 2 lb., 10 lb., tons, and things of that 
order and size—the attraction of one to another is exceedingly small. Com¬ 
pared with the attraction or repulsion of one magnetized body on another, 
or with one electrified body upon another, it is almost but not quite like 
comparing the infinitesimal with the finite. No one except one who was 
prepared for the question would suppose that there was any attraction at 
all between these two lumps of chalk, but one can readily show that there 
is a force, and a large one, between this electro-magnet and this lump cf 
iron. But, while gravitation is thus exceedingly small as compared with 
electrical, and magnetic forces, it has two characteristics which they do not 
share with it. The one is that it is always an attraction—one never hears 
of the “ repulsion ” of gravitation—and the other is it is quite universal. 
Newton’s law says that every particle of matter in the universe attracts 
every other particle, and the chemist and the astronomer of yesterday would 
both have said that he (the one) had tested this law for small masses, and that 
he (the other) had tested it for large ones. I do not think that the chemist 
would have hesitated a moment in asserting that as far as he was concerned 
the law was exact ; whilst the astronomer, who thought of the triumphs of 
the production of cometary returns—of the discovery of Neptune owing to 
the improper behaviour of Uranus, of the general and proverbial accuracy of 
astronomical occurrences — would not have hesitated long, though, as we 
shall see, there was just one thing which might have caused him to pause. 
But, whilst Newton gave a law of gravitation, he gave no explanation 
of it. He asserted it as a fact, which he derived from a mathematical 
analysis of the orbits which celestial bodies follow—that for them to follow 
these orbits “ every particle of matter in the universe must attract every 
other particle ” ; but he did not attempt to say why it did so. And neither 
since has any satisfactory explanation been proposed, though in that respect 
it is only on all-fours with electric and magnetic forces, which are similarly 
asserted but unexplained. Attempts have certainly been made (notably 
Le Sage’s), but with no acceptance or success, until Einstein proposed his 
theory, which is based upon relativity and the principle of equivalence. 
