386 | SCIENCE. 
and that physicists, at least, are tending 
towards a general agreement as to the 
nature of the Jaws in which they embody 
their discoveries, of the explanations 
which they seek to give, and of the hy- 
potheses they make in their search for ex- 
planations. 
I propose to ask you to consider the terms 
of this agreement, and the form in which, as 
it appears to me, they should be drawn up. 
The range of the physicist’s study con- 
sists in the visible motions and other sen- 
sible changes of matter. The experiences 
with which he deals are the impressions on 
his senses, and his aim is to describe in the 
shortest possible way how his various senses 
have been, will be, or would be affected. 
His method consists in finding out all 
likenesses, in classing together all similar 
events, and so giving an account as concise 
as possible of the motions and changes ob- 
served. His success in the search for like- 
nesses and his striving after conciseness of 
description lead him to imagine such a con- 
stitution of things that likenesses exist even 
where they elude his observation, and he is 
thus enabled to simplify his classification 
on the assumption that the constitution 
thus imagined is a reality. He is enabled 
to predict on the assumption that the like- 
nesses of the future will be the likenesses of 
the past.. 
His account of Nature, then, is, as it is 
often termed, a descriptive account. 
Were there no similarities in events, our 
account of them could not rise above a mere 
directory, with each individual event en- 
tered up separately with its address. But 
the similaritics observed enable us to class 
large numbers of events together, to give 
general descriptions, and indeed to make, 
instead of a directory, a readable book of 
science, with laws as the headings of the 
chapters. 
These laws are, I believe, in all cases 
brief descriptions of observed similarities. 
[N. 8S. Vou. X. No. 247. 
By way of illustration let us take two or 
three examples. 
The law of gravitation states that to each 
portion of matter we can assign a constant 
—its mass—such that there is an accel- 
eration towards it of other matter propor- 
tional to that mass divided by the square 
of its distance away. Or all bodies re- 
semble each other in having this accel- 
eration towards each other. 
Hooke’s law for the case of a stretched 
wire states that each successive equal small 
load produces an equal stretch, or states that 
the behavior of the wire is similar for all 
equal small pulls. 
Joule’s law for the heat appearing when 
a current flows in a wire states that the 
rate of heat development is proportional to 
the square of the current multiplied by the 
resistance, or states that all the different 
cases resemble each other in having 
H+C’R¢# constant. 
And, generally, when a law is expressed 
by an equation, that equation isa statement 
that two different sets of measurements are 
made, represented by the terms on the two 
sides of the equation, and that all the dif- 
ferent cases resemble each other in that the 
two sets have the constant relation expressed 
by the equation. Accurate prediction is 
based on the assumption that when we have 
made the measurements on the one side of 
the equation we can tell the result of the 
measurements implied on the other side. 
If this is a true account of the nature of 
physical laws, they have, we must confess, 
greatly fallen offin dignity. No long time 
ago they were quite commonly described as 
the Fixed Laws of Nature, and were sup- 
posed sufficient in themselves to govern the 
universe. Now we can only assign to them 
the humble rank of mere descriptions, often 
tentative, often erroneous, of similarities 
which we believe we have observed. 
The old conception of laws as self-suffic- 
ing governors of Nature was, no doubt, a 
