To use an illustration of the former suggested by the book of Leviticus. In very early times it was noticed that animals that had both the characteristics of being cloven-hoofed and of chewing the cud were good for food. A new animal is discovered having those characteristics. It is argued from the general principle laid down that this new animal is good for food, and the matter is verified by experiment. There are the four distinct steps: observation of the facts, drawing a principle from the comparison of the facts, deducing as to the particular case, verifying. The result is, of course, not only a classifying of the particular case, but also the extension of the principle. So with the generalization of the law of gravitation. Numberless facts were observed with the greatest care; from them the principle was generalized; from that again deductions were made as to particular cases; and the results were verified. But though the steps of the process are the same in both instances, yet what a vast difference between them! Take the first step, the observation of facts. All that the thought of the earlier age could do was to note a few striking resemblances and differences among the animals that roamed the neighboring forests. What could be done in the later age, ay, what the scientific temper of the age demanded, was the most rigidly careful examination of multitudes of facts; examination by a trained mind and with all the improved appliances which science and art had given to the world, and then submitted to the searching scrutiny of other trained minds, with like appliances. Or take the last step, verification. In one case it meant finding the effect upon the taste and upon the health. In the other, what it meant may be judged from the account we have of one of Newton's investigations. In applying his hypothesis of gravitation (it was only a hypothesis then) to the motion of the moon, there was a very slight divergence, about two feet a minute, between the time of the revolution of the moon in its orbit, as he calculated it and as he observed it. He was not satisfied until, _eighteen years after_, on account of an improvement made in the method of taking observations, he was able to obtain what he regarded as a verification. And so what we learn from the history of science is the gradual _development_ of scientific method. Scientific method in the work of Hipparchus meant a very different thing from the scientific method of the Chaldeans. Very different still is the scientific method of studying the heavens to-day. So to an even greater degree is there a difference between the scientific method of studying the earth to-day and as our fathers studied it. It is not merely the multitude of facts that we have learned, nor the marvelous instruments that we have made to aid us in our observations; it is also, and by no means least, this--that men all these centuries have been _learning_ to observe, to reason, and to verify.