778 POPULAR SCIJENCE MONTHLY. 



is very plain reasoning in a circle from wliicli Heaven defend 

 us all, and particularly our neighbor ! The only evidence of this 

 sort that is confirmatory must be a posteriori. If, from the con- 

 sideration of things in general, a universal principle is brought to 

 light, then the agreement of subsequently discovered or subse- 

 quently considered phenomena with this principle is strong evi- 

 dence in favor of the probable truth of the principle itself. These 

 considerations apply nowhere perhaps with greater force than in 

 the investigation of gravitation. The problem is practically where 

 Newton left it, unless indeed it has been rendered even more dif- 

 ficult by our inheritance of debarring ideas. The concluding 

 chapter of the first book is wisely given to a psychological study 

 of the current view of gravitation. The basic principle is not far 

 to seek. It lies in the doctrine that gravitation is proportional to 

 mass, and that mass is constant. With this conception firmly 

 fixed in the mind, the interpretation of the phenomena of gravi- 

 tation becomes a foregone conclusion. Yet the present authors 

 show that this conception is far from being a necessary deduction 

 from the facts of gravitation, is indeed probably a false deduc- 

 tion. Their experiments and reasoning lead them to believe, and 

 the belief is supported by other observers, that either mass is not 

 constant, or else that gravitation depends upon other factors than 

 simple mass and distance. This alternative, however, as we 

 shall see, is merely verbal. 



Many of the psychological considerations in this first book are 

 very obvious, yet they are none the less necessarj'-, and we com- 

 mend most highly the skillful vestibule which Mr. Singer and Mr. 

 Berens have constructed to their new Temple of Truth. 



The second book has to do with first principles. These the 

 authors find to be four persistence, resistance, reciprocity, and 

 equalization. The four primary principles represented by these 

 terms are at the basis of the whole work, and of their truth and 

 adequacy the authors express themselves as having no misgivings 

 whatever. They believe that the entire phenomena of the visible 

 universe can be explained by referring them to these principles. 



The principle of persistence grows out of a consideration of 

 Newton's first law of {notion : " Every body perseveres in its state 

 of rest, or of uniform motion in a right line, unless it is compelled 

 to change that state by forces impressed thereon." This is, of 

 course, only the principle of inertia, which is simply another, and 

 it has always seemed to us an unnecessary, statement of the prin- 

 ciple of causation. Nothing happens without a cause. It is per- 

 haps well to emphasize this principle when fighting superstitions 

 and other hobgoblins ; but in an intelligent world it may be 

 taken for granted as one of those primary conditions of thought 

 which need neither statement nor discussion. The habit of speak- 



