i 5 o THE POPULAR SCIENCE MONTHLY. 



ceit hatched under my own time-tonsured pate, but genuine wisdom 

 which I have simply borrowed from an old, clear-headed fellow, who 

 lived and died a long while ago Leonhard Euler. If you will read 

 his seventy-fourth letter to a German princess, written on the 11th day 

 of November, 1760, you will find it all set forth at great length. In 

 reading it you must bear in mind, though, that in Euler's time the im- 

 ponderables, as they were then called, were not so distinctly known or 

 believed to be modes of motion as they are now. And you must also 

 remember that he was writing to a princess who probably knew more 

 about madrigals and operatic airs than about scientific terms, in conse- 

 quence whereof his exposition became a little diffuse. If, however, 

 you should reject eld Euler's reasoning as "belonging to a past age of 

 thought," which, I see, is one of your favorite ways of getting rid of 

 irrefutable truths, I may refer you to a gentleman who is yet among 

 the living Hermann Helmholtz. You will find what he has to say on 

 the "matter in hand," on the third and fourth pages of his first essay, 

 " Ueber die Erhaltung der Kraft " (not included in the collection of 

 his essays). 



Now, hombre querido (I am still preaching), if after this you will 

 carefully read again the first twelve chapters of my book, you will 

 probably find that they are somewhat less absurd than you fancied they 

 were. But you will say, no doubt in fact, you do say, though not 

 in so many words that all this is mere speculative trash, in which the 

 man of science has no concern. One of my reviewers in the New 

 York " Critic " whom I at one time suspected, perhaps unjustly, from 

 certain peculiarities of his phraseology, and from the fact that, like 

 yourself, he sneers at me for having "wasted" two long chapters on 

 transcendental geometry, of having had oral confabulations with you, 

 in which the mouth of the speaker was not and could not be applied 

 to the ear of the listener disposes of my discussion of the relation of 

 the mechanical theory to the laws of thought by the following oracu- 

 lar dictum (a travesty of a saying of Carlyle) : "A sound digestion has 

 little self -consciousness of the operations of the stomach; the sound 

 thinker gives himself little uneasiness respecting the laws of thought." 

 I can not stop, at this moment, to show you how and why a little 

 knowledge of the laws of thought is useful to the physicist and mathe- 

 matician. I shall come to that by-and-by, when I have considered 

 what you say about the kinetic theory of gases and space of an indefi- 

 nite number of dimensions. For the present I only want to tell you 

 how I ventured upon the audacity of intruding the theory of cognition 

 into the science of physics. 



In Europe, as well as in this country, there are certain idle fellows 

 who, during the first half of the present century, for want of more 

 useful occupation, took to tracing the ramifications of forms of speech, 

 and finally got to digging for their roots. These absurd persons 

 abound chietly in Germany, where, as you know, the people are always 



