excessive subjectivity that mostly prevailed. This reaction found expression in men like Archimedes and Hipparchus, and under their influence science had apparently attained a foothold when the intolerance of a religious supremacy, which held sway over Europe during the dark ages, banished it from Christendom, and thus became debited to humanity for a thousand years of stagnation.
Not until the time of Copernicus was there any new impetus to research. The appeal to objective facts was, then, although in a small degree, made the avowed basis of scientific speculation. Then began the progress of knowledge which, with constantly accelerating speed, has continued to our times, and which, we trust, may not again be slackened.
The experience of our predecessors teaches us a lesson that we ought not soon to forget. In the practical affairs of life, experience is considered the best guide. The man who learns nothing from its teachings does not succeed. It is no less important in the life of the race. If we learn nothing from others' failures, we can expect to learn nothing from their successes.
The attempts to construct the universe a priori are analogous to the attempts to construct a perpetual motion. As long as there was no direct reference to experience, it appeared by no means improbable that a machine might be so constructed that, when once set in motion, it would never stop. Any one, who has had any experience with a class in mechanics, knows how crude are the notions they possess with reference to the very elements of physics. It is safe to assert that today not one person in ten possesses a thorough certainty of belief in the validity of the third law of motion, namely, that action and reaction are always equal; or, to put it in more familiar language, that, when a horse pulls a wagon, the wagon pulls the horse equally hard. It is not until correct mechanical notions become wrought into the very warp and woof of one's mental fabric that his deductions from them may fairly be presumed to be correct. Witness the absurd notions current only a very few years ago as to the possibility of constructing a "motor" that should, in some mysterious way, evolve force enough from a pint of water to propel a train of cars from Philadelphia to New York! Mechanics went on inventing "perpetual motions," but, however plausible they looked, somehow or other they wouldn't work. Do away with friction to the greatest possible extent; overcome one obstacle here and another there; increase the time the machine would run indefinitely—still, to a state of equilibrium would it come at last. From these empirical observations mechanics learned, what philosophers might have learned years ago, that experience affords the only sure test of the validity of our notions.
After numberless failures had taught the empirical lesson that attempts to construct a perpetual motion were fruitless, the gradual development of the law of the conservation of energy showed the reason
