70 THE POPULAR SCIENCE MONTHLY. 



1. Two Kinds of Impossibles. — On the very threshold of my 

 subject I am met by the objection that " many things far more 

 wonderful, and, before their realization, seemingly far more im- 

 possible, than flying-machines, have, nevertheless, actually come 

 to pass. Then why not this also ? He is a bold man that declares 

 anything impossible in this age of rapid progress and startling 

 inventions." I answer : True enough, many wonderful and appar- 

 ently impossible things have indeed come to pass ; and that, too, 

 in spite of the adverse predictions of some rash scientists. But 

 there are two kinds of impossibles — the seeming and the real. 

 The seeming impossibles we believe to be impossible, only be- 

 cause we do not yet understand the principles involved in the 

 problem, and therefore can not conceive the conditions necessary 

 for their successful application. Such are all the cases which 

 most readily occur to the mind as triumphs of science — such, for 

 example, as the locomotive, the telegraph, the telephone, etc. The 

 real impossibles, on the contrary, we know to be such, because we 

 see clearly through all the principles involved in the problem 

 and the limits of their possible application. Of this kind are the 

 iwohlem of a perpetual-motion machine, and of a self-supiiorting 

 arch of indefinite length. Now observe — that, of these two kinds 

 of impossibles, to the unreflecting the seeming are far the more 

 impossible and wonderful. In fact, to most people the real impos- 

 sibles do not seem impossible, or wonderful, or even difficult at 

 all. Hence, in every age and country we find men who waste their 

 lives in vain attempts to make perpetual-motion machines. So, 

 also, in regard to the indefinite arch. Most people do not see at 

 once why an arch of any length should not support itself if only 

 it be big and strong in proportion to its length. 



Let me stop a moment to illustrate this by an anecdote, I 

 remember many years ago meeting a traveling agent of a Rem- 

 ington bridge (a wooden suspension-bridge), who had with him 

 for exhibition a small model which, when set up, was about 

 twenty feet long, and had stringers about as big as my finger. 

 This little model not only sustained itself, but, in addition, the 

 weight of a stout looker-on — " a fat and greasy citizen " — twenty 

 times as heavy as the bridge itself. " Now," said the plausible 

 agent, " if you increase the size and strength of the stringers in 

 proportion as you increase the length of your bridge, it is evident 

 that a bridge of this pattern, of any length, will not only sustain 

 itself, but twenty times its own weight in the form of loaded 

 wagons," Most of those who heard it accepted his reasoning as 

 irrefutable. Of course, every engineer knows that this is not 

 true. For, while the iveight of the bridge increases as the cuhe of 

 the diameter of all its parts, the strength of the stringers increases 

 only as the square of their diameter. In increasing the size in 



