THURSDAY, JULY 11, ror. 
THE PHYSICAL CONSTITUTION OF THE 
EARTH. 
Some Problems of Geodynamics. Being an Essay 
to which the Adams Prize in the University of 
Cambridge was adjudged in 1911. — By Prof. 
A. E. H. Love, F.R.S. Pp. xxviiz-180.- (Carn- 
bridge: University Press, 1911.) Price 12s. 
net. 
HIS book consists of a number of discussions 
important of geophysics. 
The nature and bearing of these will be best un- 
derstood if we cast a rapid glance at the modern 
history of the subject. Fifty years ago the theory 
of the constitution of the earth was generally re- 
garded as complete, in the sense that almost 
of questions 
everything was thought to be known that was 
in the nature of things ascertainable. The 
external shape and the distribution of density in 
the interior were assumed to be such as are con- 
sistent with primitive, or indeed with present, 
fluidity (except for a superficial crust); and a cer- 
tain reasonable law of density, viz., that of Laplace, 
was regarded as, if not actually demonstrated. at 
all events highly probable. The theory was a 
monument of mathematical skill, and had indeed 
evoked mathematical methods which had proved 
to have an ever-increasing value in other fields; 
but as a speculative mine it was held to be prac- | 
tically worked out. 
The popular view as to the actual internal 
fluidity of the earth had indeed been questioned 
by Hawkins. His the 
phenomena of precession and nutation, were taken 
up by Thomson in 1862, from which epoch we 
may date the beginnings of the modern revolu- 
tion in the subject. This particular line of reason- 
ing proved, however, to be more abstruse than 
arguments, based on 
was at first recognised, and had to be revised at 
a later period; it is scarcely referred to in the work 
now under notice. line of ‘attack, 
initiated by Thomson, proved more convincing. 
He pointed out that the existence of tides which 
can be observed implies a high degree of effective 
A quantitative 
Another 
rigidity ir the earth as a whole. 
NATURE 
estimation is difficult, owing to the unavoidable | 
imperfections of tidal theory; but the amplitudes 
which the long-period tides at all events (lunar 
fortnightly and solar semi-annual) would have on 
an absolutely rigid earth may be regarded as 
known, and the comparison with observation ap- 
471 
This conclusion is supported by the direct ob- 
servation of the lunar disturbance of gravity, 
suggested originally by Thomson, attempted by 
G. H. and H. Darwin, and at length carried out 
successfully, with amazing skill and perseverance, 
by Hecker. The perplexing result obtained by 
Hecker at Potsdam, and found also to a some- 
what lesser degree by Orloff in similar observa- 
tions at Dorpat, that the yielding appears to be 
sensibly greater in the N.-S. than in the E.-W. 
direction, has excited much speculation. Some re- 
lation to the earth’s axis of rotation is at first sight 
indicated ; and Prof. Love has accordingly devoted 
an important section of his researches to a dis- 
cussion. of the theoretical effects of rotation and 
of the correlated ellipticity of the meridian. His 
calculations show, as might have been anticipated, 
that no difference at all comparable with that which 
is observed is to be accounted for in this way. The 
only remaining suggestion at present in the field 
is that the effect is due to the direct attraction 
of the Atlantic tidal wave, and to the deformation 
produced by its weight in the neighbouring 
regions of the earth. 
The theoretical calculation of the tidal deforma- 
tion of an elastic globe, which forms one of Thom- 
son’s most massive contributions to mathematical 
method, was naturally based on certain simplify- 
ing assumptions, among which was that of in- 
compressibility. Prof. Love contributes the very 
interesting extension to the case of compressible 
solids. The result shows that the compressibility 
will increase the amount of yielding, as is required 
by general principles; but it appears that, on any 
reasonable supposition applicable to the case of 
the earth, the difference, though appreciable, is 
not such as to affect the general validity of 
inferences based on former results. .In this con- 
nection the author contributes another chapter to 
an important problem of mathematical physics by 
investigating the periods of free vibration of a 
homogeneous, but compressible, and gravitating 
globe. 
The same analysis enables Prof. Love to discuss 
very fully the question, raised a few years ago by 
Jeans, as to the gravitational stability of the 
earth. It is conceivable, and indeed proved to be 
possible, that a gravitating mass might be in 
equilibrium in such a condition that the loss of 
gravitational energy consequent on some par- 
ticular type of deformation might exceed the 
gain of elastic energy, in which case there would 
of course be instability. The question arises 
pears to show distinctly that the earth itself does | whether the actual large-scale irregularities of the 
as a matter of fact yield to tidal distorting force, 
but only to such an extent as if its rigidity were 
comparable with that of steel. 
NO. 2228, vor. 89] 
, earth’s 
surface, which have been very fully 
analysed by Prof. Love in a previous publication, 
may not be, as it were, the record of a catastrophe 
U 
