Now there are some terrestrial phenomena which we might expect to respond in greater or less degree to the sun’s changes of condition with respect to spots. We cannot doubt that the emission both of light and of heat is affected by the presence of spots. It is not altogether clear in what way the emission is affected. We cannot at once assume that because the spots are dark the quantity of sunlight must be less when the spots are numerous; for it may well be that the rest of the sun’s surface may at such times be notably brighter than usual, and the total emission of light may be greater on the whole instead of less. Similarly of the emission of heat. It is certain that when there are many spots the surface of the sun is far less uniform in brightness than at other times. The increase of brightness all round the spots is obvious to the eye when the sun’s image, duly enlarged, is received upon a screen in a darkened room. Whether the total emission of light is increased or diminished has not yet been put to the test. Professor Langley, of the Alleghany Observatory, near Pittsburg, U.S., has carefully measured the diminution of the sun’s emission of light and heat on the assumption that the portion of the surface not marked by spots remains unchanged in lustre. But until the total emission of light and heat at the times of maximum and minimum has been measured, without any assumption of the kind, we cannot decide the question. More satisfactory would seem to be the measurements which have been made by Professor Piazzi Smyth, at Edinburgh, and later by the Astronomer Royal at Greenwich, into the underground temperature of the earth. By examining the temperature deep down below the surface, all local and temporary causes of change are eliminated, and causes external to the earth can alone be regarded as effective in producing systematic changes. “The effect is very slight,” I wrote a few years ago, “indeed barely recognizable. I have before me as I write Professor Smyth’s sheet of the quarterly temperatures from 1837 to 1869 at depths of 3, 6, 12, and 24 French feet. Of course the most remarkable feature, even at the depth of 24 feet, is the alternate rise and fall with the seasons. But it is seen that, while the range of rise and fall remains very nearly constant, the crests and troughs of the waves lie at varying levels.” After describing in the essay above referred to, which appears in my “Science Byways,” the actual configuration of the curves of temperature both for seasons and for years, and the chart in which the sun-spot waves and the temperature waves are brought into comparison, I was obliged to admit that the alleged association between the sun-spot period and the changes of underground temperature did not seem to me very clearly made out. It appears, however, there is a slight increase of temperature at the time when the sun-spots are least numerous.