1118 REPORT—1885. 
Survey, tend to show that the earth is solid to its core, and that the geological 
hypothesis of a fluid interior is untenable. They have been analysed by Professor G. 
H. Darwin, with a view to the determination 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 51, the latter 
somewhat less than;%,; but as new and more exact Gata became available, the 
walues derived from these two essentially independent sources became more and 
more accordant, and they now nearly agree in the value 515. 
Asa part of the pendulum operations, a determination of the length of the 
seconds’ pendulum was made at Kew by Major 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 3971393 inches. Thus, while the French took for 
their unit of length the ten-millionth part of the earth’s 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 accuracy; 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 satis- 
factory results can be obtained in a few hours instead of many days, the boon to 
science will be very great. 
The trigonometrical operations fix with extreme accuracy two of the co-ordinates 
—the latitude and longitude—which define the positions of the principal stations ; 
‘but the third co-ordinate, the height, is not susceptible of being determined by such 
operations with anything like the same degree of accuracy, because of the variations 
of refraction to which rays of light passing through the lower strata of the atmo- 
sphere are liable, as the temperature of the surface of the ground changes in the 
course of the day. In the plains the apparent height of a station ten to twelve 
miles from the observer has been found to be upwards of 100 feet greater in the 
cool of the night than in the heat of the day, the refraction being always positive 
when the lower atmospheric strata are chilled and laden with dew, and negative 
when they are rarefied by the heat radiated from the surface of the ground. At 
hill stations the rays of light usually pass high above the surface of the ground, and 
the diurnal variations of refraction are comparatively immaterial, and very good 
results are obtained by the expedient of taking the vertical observations between 
reciprocating stations at the same hour of the day, and as nearly as possible at the 
time of minimum refraction; but in the plains this expedient does not usually 
