We may leave the problem in this state of uncertainty for the present, because it will appear later that astronomy has some evidence to give on the question. CHAPTER III _Exploring in Time_ We have explored space to the furthest depths to which our telescopes can probe; we have explored into the intricacies of the minute structures we call atoms, of which the whole material universe is built; we now wish to go exploring in time. Man’s individual span of life, and indeed the whole span of time covered by our historical records—some few thousands of years at most—are both far too short to be of any service for our purpose. We must find far longer measuring rods with which to sound the depths of past time and to probe forward into the future. Our general method will be one which the study of geology has already made familiar. Undeterred by the absence of direct historical evidence, the geologist insists that life has existed on earth for millions of years, because fossil remains of life are found to occur under deposits which, he estimates, must have taken millions of years to accumulate. As he digs down through different strata in succession, he is exploring in time just as truly as the geographer who travels over the surface of the earth is exploring in space. A similar method can be used by the astronomer. We find some astronomical effect, quality, or property, which exhibits a continual accumulation or decrease, like the sand in the bottom or top half of the hour-glass; we estimate the rate at which this increase or decrease is occurring at the present moment, and also, if we can, the rate at which it must have occurred under the different conditions prevailing in the past. It then becomes a question, perhaps of mere arithmetic, although possibly of more complicated mathematics, to estimate the time which has elapsed since the process first started. THE AGE OF THE EARTH The method is well exemplified in the comparatively simple problem of the age of the earth. The first scientific attempt to fix the age of the earth was made by Halley, the astronomer, in the year 1715. Each day the rivers carry a certain amount of water down to the sea, and this contains small amounts of salt in solution. The water evaporates and in due course returns to the rivers; the salt does not. As a consequence the amount of salt in the oceans goes on increasing; each day they contain a little more salt than they did on the preceding day, and the present salinity of the oceans gives an indication of the length of time during which the salt has been accumulating. “We are thus furnished with an argument,” said Halley, somewhat optimistically, “for estimating the duration of all things.” This line of argument does not lead to very precise estimates of the earth’s age, but calculations based on modern data suggest that it must be many hundreds of millions of years.