368 Intelligence and Miscellaneous Articles. 



show the existence of volcanic vents in that region in an early part of 

 the Lower Cambrian period. The volcanic accumulations were 

 covered conformably by the Conglomerate and succeeding Cambrian 

 groups ; but the same kind of tuffs continued to be ejected after the 

 deposition of the Conglomerate. At a later time this thick con- 

 formable succession of beds was plicated, and underwent a partial 

 metamorphism, -whereby some of the fine tuffs and shales were con- 

 verted into sericite-schists. Subsequently a mass of granite rose 

 through one side of the fold, accompanied by elvans of spheru- 

 litic quartz-porphyry, whereby a second, different and feebler 

 kind of metamorphism was induced. The last episode was that of 

 the diabase dykes, which, crowded together in the granite, sug- 

 gest that the granite boss stands on an old line of weakness and 

 of escape for eruptive material from the interior. 



LIV. Intelligence and Miscellaneous Articles. 



To the Editors of the Philosophical Magazine and Journcd. 

 Gentlemen, 



THE anomalies and confusions which Mr. Close points out in the 

 use of the term " Force " (Phil. Mag. April, p. 248) appear to 

 me mainly to arise from the very unfortunate definition which he, 

 in common with other modern physicists, insists on applying to the 

 word. Force proper, he says, is " the rate of change of momentum ;" 

 and this he would substitute for the old definition, common to all 

 text-books on Mechanics, that a force is a cause of motion. Now 

 if there is one thing which seems clear, in the framing of termino- 

 logy, it is that where a well-known word receives a scientific defi- 

 nition, that definition should be in as close accordance as possible 

 with its ordinary use. In the light of this rule, the new definition 

 of force is perhaps one of the worst ever constructed. For, on 

 that definition, force belongs to the science of Dynamics alone. 

 There is no force acting between a weight and the table it lies 

 upon ; no force acting between the rim of a fly-wheel and its boss ; 

 no force acting between a locomotive and a train, if the speed is 

 uniform ; and so on. It is clear that we must either give up the 

 new definition, or re-construct all our old ideas on the subject; 

 with a certainty of importing that confusion to which Mr. Close's 

 letter bears such striking witness *. 



Following the old definition, the mode of generation of the new 

 is obvious enough. The effect of a force, where it acts freely, is to 

 increase momentum ; and as causes are measured by their effects, 

 it follows that we may measure forces by calculating the rate of 



* Mr. Close contributes something of his own to this confusion when, 

 in p. 251, he speaks of F as measured in pounds. How a rate of in- 

 crease of momentum can be expressed in pounds it does not seem easy to 

 discover. 



