640 PROFESSOR PIAZZI SMYTH ON THE 
at large, that that level effect was produced by the afternoon Sun shining on, 
and expanding, the rocky western side of the Calton Hill. Wherefore also, 
said various wise men, “ the rock foundation of the Edinburgh Observatory is 
just about the worst sort of foundation that could possibly have been selected 
for an accurate, meridian, Astronomical Observatory.” 
But on examining the dates and configurations of the curve representing 
each annual progress of the level fluctuations, and comparing them with the 
similar points of the annual temperature curves of the several rock Thermo- 
meters,—I was presently able to demonstrate, that what particular thing or 
matter the heat was acting on to disturb the instrument’s level,—so far from 
being any rock-surface, more than 200, or even only 20, feet off, must be 
within less than 5 feet from the instrument itself. And it has been experi- 
mentally proved since then to depend on the abnormal expansion of the 
mountings of the instrument within its own chamber. While the grand 
porphyry rock of the Calton Hill has at the same time been shown to be fully 
worthy of supporting a much grander and more celebrated Observatory than 
has yet been erected upon it. 
Again, in following years, these rock thermometer observations were 
discussed by Sir WILLIAM THOMSON, and by aid of his suggestive philosophy and 
powerful analysis made to indicate the beginnings of some vast and soul- 
elevating results; such as 
First, the amount of the Earth’s central heat which is still year by year 
escaping through the surface and radiating into space ; and 
Second, setting forth something as to the age of that heat; or the time 
which has elapsed since the whole earth was of a life-forbidding, because light- 
emitting, temperature. Of these high reasonings the Society has already heard 
at various times from that eminent philosopher, and it is hoped will further 
hear from him directly again and again. 
More than thirty years had thus passed by. Our stock of observed 
temperature facts of the Calton Hill was always growing, simply because we 
were an Observatory, and not any other kind of institution, however estimable ; 
and presently a third supplementary useful purpose appeared, quite different 
from any of the former, and fraught with practical applications for the future, 
of no ordinary kind. 
What is so important to both the business and pleasures, the life and the 
glories of this country of ours, as the seasons of summer and winter ; and who 
does not know their intimate dependence on the physical nature of the Sun, 
combined with the grandly ordered angle of the Obliquity of the Ecliptic, the 
rotation of the Earth on its axis, and the whole globe’s envelopment in a most 
composite atmosphere, of permanent gases and watery vapour! But then again 
who does not also know, by even terrific personal experience, that there are 
