COMPLETION OF THE NEW ROCK THERMOMETERS. 641 
occasionally summers in this country avd summers; winters also and winters ; 
though why or wherefore, is still a scientific mystery. 
Taking up then that particular subject, in 1869, by means of our accumu- 
lated years of observation of the rock-thermometers, I tried a simple method 
of eliminating the effects of the ordinary seasons; and then examined closely 
the residual quantities. They might of course be expected to indicate some 
variations with time; and the one all important question for the hopes of 
science was, would it be regular, or irregular ? 
Great therefore was my joy when the result said immediately, for all the 
larger variations, REGULAR! Regular even to a surprising degree, for this was 
the order that came out ;—Every eleven years nearly (and this three times 
over in succession during the limits of our observations, so that there could be 
no mistake about it), a wave of heat had struck the earth’s surface from with- 
out (not from within, as proved by the dates being later and later according to the 
depth of each thermometer) ; and between those most signal eleven year heat 
epochs, there was no other hot wave of a supra-annual order, which could 
compare with what occurred then, for either size or intensity.* Their dates 
were 1846, 1857 and 1868. 
That was the first law of occurrence deduced by the inquiry; and the 
second, approaching to something like a cause, was—that each of those dates 
was found to mark the beginning, the well commenced beginning too, of 
precisely the most intense period of a well-known cyclical habitude of Solar 
activity ;—acting doubtless, both by its increased heat and other radiant 
emanations, though chiefly observable to man by an accompanying optical 
phenomenon, “the Sun-spots.”t And, after that was settled, then came the 
third law of occurrence, viz. that close on either side of the heat wave, or 
more exactly 1:7 year therefrom, came a specially cold trough. Sometimes the 
preceding, Sometimes the following of these cold waves was the severer of the 
two ; but the curious rule of two was always kept up at every successive 
cycle ; and for reasons most probably of terrestrial physics which are exceed- 
ingly interesting, but too long, and far from our subject, to be entered on here. 
Practically confident then that this new mode of treating the accumulated 
observations of our rock thermometers, had helped me to a most important 
* These epochal waves of heat may apparently come to a head at any time in the course of a 
year; but are never so powerful as to destroy altogether the ordinary summer or winter. If therefore 
one, of any cycle, occur just at mid-winter, it merely makes it an unusually mild winter; not a season 
as hot as, or hotter than, an ordinary summer. 
+ These Sun-spots have been proved, by Professor Lanatey and others, to have no power in 
themselves to produce any notable alteration in the quantity of the Sun’s radiant heat practically felt 
by the earth and its atmosphere ; but bear about the same relation to it, that the cinders dropping into 
the ash-pit of a steam engine’s fire-place, do to the amount of mechanical work being performed by the 
engine at the moment; and if the engine itself is not observable by us, it is well to note and reason 
on the number and size of the red-hot cinders, and the rate at which they fall. 
