682 THE POPULAR SCIENCE MONTHLY. 



excessive subjectivity that mostly prevailed. This reaction found ex- 

 pression in men like Archimedes and Ilipparchus, and under their influ- 

 ence 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 re- 

 search. 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 exjjerience, it appeared by no means imjDrobable 

 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 to- 

 day not one person in ten possesses a thorough certainty of belief in 

 the validity of the third law of motion, namely, that action and reac- 

 tion 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 philoso- 

 phers 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 at- 

 tempts to construct a perpetual motion were fruitless, the gradual de- 

 velopment of the law of the conservation of energy showed the reason 



