486 
swung in his notable operations which extend from a little below 
the equator to within ro” of the pole. Captain Basevi had nearly 
completed the operations in India, and had taken swings at a 
number of the stations of the Great Arc and at various other 
points near mountain ranges and coast lines, when he died of 
exposure in 1871 at a station on the high table-lands of the Him- 
alayas, while investigating the force of gravity under mountain 
ranges. Major Heaviside swung the pendulums at the remaining 
Indian stations, then at Aden and Ismailia on the way back to 
England, and finally at the base station, the Kew Observatory. 
Afterwards they and a third pendulum were swung at Kew and 
Greenwich by Lieutenant-Colonel Herschel, who took all three 
to America, swung them at Washington, and then handed them 
over to officers of the United States Coast Survey, by whom they 
have been swung at San Francisco, Auckland, Sydney, Singa- 
pore, and in Japan. 
The pendulum operations in India have been successful in 
removing from the geodetic operations the reproach which had 
latterly been cast on them, that their value has become much 
diminished since the discovery that the attraction of the Hima- 
layan mountains is so much greater than had previously been 
suspected, that it may have materially deflected the plumb-line 
at a large number of the astronomical stations of the Great Arc, 
and injuriously influenced the observations. Everest considered 
the effects of the Himalayan attraction to be immaterial at any 
distance exceeding sixty miles from the feet of the mountains ; 
but in his days the full extent and elevation of the mountain 
masses was unknown, and their magnitude was greatly under- 
estimated. Afterwards, when the magnitude became better 
known, Archdeacon Pratt of Calcutta, a mathematician of great 
eminence, calculated that they would materially attract the plumb- 
line at points many hundred miles distant; he also found that 
everywhere between the Himalayas and the ocean, the excess of 
density of the land of the continent as compared with the water 
of the ocean would combine with the Himalayan attraction and 
increase the deflection of the plumb-line northwards, towards 
the great mountain ranges, and that under the joint influence of 
the Himalayas and the ocean the level of the sea at Kurrachee 
would be raised 560 feet above the level at Cape Comorin. 
But as a matter of fact the Indian are gave a value of the 
earth’s ellipticity which agreed sufficiently closely with the values 
derived from the arcs measured in all other quarters of the globe, 
to show that it could not have been largely distorted by deflec- 
tions of the plumb-line ; thus it appeared that whereas Everest 
might have slightly underestimated the Himalayan attraction, 
Pratt must have greatly overestimated it. His calculations were 
however based on reliable data, and were indubitably correct. 
For some time the contradiction remained unexplained, but 
eventually Sir George Airy put forward the hypothesis that the 
influence of the Himalayan masses must be counteracted by some 
compensatory disposition of the matter of the earth’s crust im- 
mediately below them, and in which they are rooted ; he suggested 
that the bases of the mountains had sunk to some depth into a 
fluid lava which he conceived to exist below the earth’s crust, and 
that the sinking had caused a displacement of dense matter by 
lighter matter below, which would tend to compensate for the 
excess of matter above. Now Pratt’s calculations had reference 
only to the visible mountain and oceanic masses, and their attrac- 
tive influences—the former positive, the latter negative—in a 
horizontal direction ; he had no data for investigating the density 
of the crust of the earth below either the mountains on the one 
hand, or the bed of the ocean on the other. The pendulum ob- 
servations furnished the first direct measures of the vertical force 
of gravity in different localities which were obtained, and these 
measures revealed two broad facts regarding the disposition of 
the invisible matter below ; first, that the force of gravity dimin- 
ishes as the mountains are approached, and is very much less on 
the summit of the highly elevated Himalayan table-lands than 
can be accounted for otherwise than by a deficiency of matter 
below ; secondly, Ihat it increases as the ocean is approached, 
and is greater on islands than can be accounted for otherwise than 
by an excess of matter below. Assuming gravity to be normal 
on the coast lines, the mean observed increase at the island sta- 
tions was such as to cause a seconds’ pendulum to gain three 
seconds daily, and the mean observed decrease in the interior of 
the Continent would have caused the pendulum to lose 24 
seconds daily at stations averaging 1,200 feet above the sea level, 
5 seconds at 3,800 feet, and about 22 seconds at 15,400 feet— 
the highest elevation reached—in excess of the normal loss of 
rate due to height above the sea. 
NATURE 
[Sept. 17, 1885 
Pratt was strongly opposed to the hypothesis of a substratum, 
or magma, of fluid igneous rock beneath the mountains; he 
assumed the earth to be solid throughout, and regarded the 
mountains as an expansion of the inyisible matter below, which 
thus becomes attenuated and lighter than it is under regions of 
less elevation, and more particularly in the depressions and con- 
tractions below the bed of the ocean. And certainly we seem 
to have more reason to conclude that the mountains emanate 
from the subjacent matter of the earth’s crust than that they are 
as wholly independent of it as if they were formed of stuff shot 
from passing meteors and asteroids ; any severance of continuity 
and association between the visible above and the invisible below 
appears, on the face of it, to be decidedly improbable. 
The hypothesis of sub-continental attenuation and sub-oceanic 
condensation of matter is supported by the two arcs of longitude 
on the parallels of Madras and Bombay; for at the extreme 
points of these ares, which are situated on the opposite coast lines, 
the horizontal attraction has been found to be not landwards, as 
might have been anticipated, but seawards, showing that the de- 
ficient density of the sea as compared with the land is more than 
compensated by the greater density of the matter under the 
ocean than of that under the land. 
While on the subject of the constitution of the earth’s crust, 
I may diaw attention to the circumstance that the tidal observ- 
ations which have been carried on at a number of points on the 
coasts of India, as a part of the operations of the Survey, tend 
to shew that the earth is solid to its core, and that the geological 
hypothesis of a fluid interior is untenable. They haye been 
analysed by Prof. G. H. Darwin, with a view to the determin- 
ation of a numerical estimate of the rigidity of the earth, and he 
has ascertained that whilst there is some evidence of a tidal 
yielding of the earth’s mass, that yielding is certainly small, and 
the effective rigidity is very considerable, not so great as that of 
steel, as was at first surmised, but sufficient to afford an important 
confirmation of the justice of Sir William Thomson’s conclusion 
as to the great rigidity. 
The Indian pendulum observations have been employed by 
Colonel Clarke, in combination with those taken in other parts 
of the globe, to determine the earth’s ellipticity. Formerly there 
was wont to be a material difference between the ellipticities 
which were respectively derived from pendulum observations and 
direct geodetic measurements, the former being somewhat greater 
than z4,, the latter somewhat less than s},; but as new and 
more exact data became available, the values derived from these 
two essentially independent sources became more and more 
accordant, and they now nearly agree in the value s4;. 
As a part of the pendulum operations, a determination of the 
length of the seconds’ pendulum was made at Kew by Mejor 
Heaviside, with the pendulum which had been employed for the 
same purpose by Kater early in the present century, when leading 
men of science in England believed that in the event of the 
national standard yard being destroyed or lost, the length might 
be reproduced at any time with the aid of a reversible pendulum. 
In consequence of this belief an Act of Parliament was passed 
in 1824 which defined the relations between the imperial and the 
seconds’ pendulum, the length of the former being to that of the 
latter—swung in the latitude of London, in a vacuum and at the 
level of the sea—in the proportion of 36 inches to 39°1393 inches. 
Thus, while the French took for their unit of length the ten- ~ 
millionth part of the earths’ meridional quadrant, the English 
took the pendulum swinging seconds in the latitude of London. 
In case of loss the yard is obviously recoverable more readily and 
inexpensively by reference to the pendulum than the metre by 
reference to the quadrant ; it is also recoverable with greater ac- 
curacy ; still the accuracy is not nearly what would now be deemed 
indispensable for the determination of a national standard of 
length, and it is now generally admitted that every pendulum has 
certain latent defects, the influence of which cannot be exactly 
ascertained. Thus the instrument cannot be relied on as 
a suitable one for determinations of absolute length ; 
but, on the other hand, so long as its condition remains 
unaltered, it is the most reliable instrument yet discovered 
for differential determinations of the variations of gravity. In 
truth, however, the pendulum is a very wearisome instrument to 
employ even for this purpose, for it has to be swung many days 
and with constant care and attention to give a single satisfactory 
determination ; thus if such a thing can be invented and perfected 
as a good differential gravity meter, light and portable, with 
which satisfactory results can be obtained in a few hours, instead 
of many days, the boon to science will be very great. 
