Progress in Science. 
[October^ 
except small cavities or vesicles filled with liquid, yield to the deforming (or 
tide generating) influences of sun and moon ? This question could only be 
answered by observation. A single infinitely accurate spirit level or plummet 
far enough away from the sea to be not sensibly affedted by the attraction of 
the rising and falling water, would enable us to find the answer. Closely 
connected with the question of the earth’s rigidity, and of as great scientific 
interest and even greater practical moment, was the question — how nearly 
accurate is the earth as a timekeeper ? and another of, at all events, equal 
scientific interest — how about the permanence of the earth’s axis of rotation ? 
Peters and Maxwell, about thirty-five and twenty-five years ago, separately 
raised the question, how much does the earth’s axis of rotation deviate from 
being a principal axis of inertia? and pointed out that an answer to this 
question was to be obtained by looking for a variation in latitude of any or every 
place on the earth’s surface in a period of 306 days. Peters gave a minute 
investigation of observations at Pulkova in the years of 1841-42, which seemed 
to indicate at that time a deviation amounting to equal to about 3-40 seconds 
of the axis of rotation from the principal axis. Maxwell from Greenwich 
found seeming indications of a very slight deviation — something less than half 
a second — but differing altogether in phase from that which the deviation 
indicated by Peters, if real and permanent, would have produced at Maxwell’s 
later time. On his (Sir William Thomson) begging Prof. Newcomb to take up 
the subject, he undertook to analyse a series of observations suitable for the 
purpose, which had been made in the United States Naval Observatory, 
Washington. A few weeks later he received from him a letter referring him 
to a paper by Dr. Nysen, of Pulkova Observatory, in which a negative con- 
clusion as to constancy of magnitude or direction in the deviation sought for 
is arrived at from several series of the Pulkova observations between the 
years 1842 and 1872, and containing a statement of his own conclusions, which 
were also negative. From the discordant charadter of these results we must 
not, however, infer that the deviations indicated by Peters, Maxwell, and 
Newcomb were unreal. On the contrary, any that fall within the limits of 
probable error of the observations ought properly to be regarded as real. 
There was in fadl a vera causa in the temporary changes of sea level due to 
meteorological causes, chiefly winds, and to meltings of ice in the polar 
regions, and return evaporations, which seemed amply sufficient to account 
for irregular deviations of from cne-half to i-20th second of the earth’s 
instantaneous axis from the axis of maximum inertia, or, as he ought rather 
to say, of the axis of maximum inertia from the instantaneous axis.” Sir 
William concluded his address by considering the variations in the earth’s 
rotational period. 
The report of the Committee for Testing Experimentally the Exactness of 
Ohm’s Law, drawn up by Prof. Clerk Maxwell, was read by Mr. Chrystal. 
The result of this investigation is described as follows: — If a conductor of 
iron, platinum, or German silver of one square centimetre in sedtion has a 
resistance of one ohm for infinitely small currents, its resistance when adted 
on by an eledtromotive force of one volt (provided its temperature is kept the 
same) is not altered by so much as the millionth of a millionth part. The 
report concludes : — “ It is seldom, if ever, that so searching a test has been 
applied to a law which was originally established by experiment, and which 
must still be considered a purely empirical law, as it has not hitherto been 
deduced from the fundamental principles of dynamics. But the mode in which 
it has borne this test not only warrants our entire reliance on its accuracy 
within the limits of ordinary experimental work, but encourages us to believe 
that the simplicity of an empirical law may sometimes be an argument for its 
exactness, even when we are not able to show that the law is a consequence 
of elementary dynamical principles.” 
One of the most important papers contributed to the sedtion was that by 
Professor Stokes on “ The Phenomena of Metallic Refledtion.” Professor 
Stokes explained that when Newton’s rings were formed between a lens and a 
