MacCulloch's System of Geology. STA 



ing lecture on hammers: " The hammer of quarry men is a 

 double truncated wedge : while nothing but extreme ignorance 

 of mathematical principles could have extended this form to 

 those of road-makers and geologists : whence their inefficiency, 

 even under an inconvenient increase of weight. The fragment, 

 in each case, is produced by exciting a vibration in an imaginary 

 lamina, through a motion too sudden to be accompanied by 

 that of the adjoining parts. It is forgotten that the commu- 

 nication of motion is not regulated by the momentum, simply, 

 of the moving body. The weight and the velocity cannot be 

 indifferently interchanged : and thence strange errors in many 

 other matters also, of daily practice. But, not to enter further 

 on this subject, suffice it, that it requires a time inversely pro- 

 portioned to the tenacity of a body, to allow it to be displaced 

 in a mass ; whence, if additional momentum be required, the 

 weight, not the velocity, must be increased. But if it is re- 

 quired to disintegrate the same body, it is the velocity that 

 must be augmented. It is by impulse that the stone is bro- 

 ken in the present case. Hence it is useless to increase the 

 weight of such a hammer, beyond that to which the hand can 

 give the greatest velocity, since there is thus a loss of power. 

 I have taken four pounds as the extreme weight ; while two, 

 or less, will, in a proper form, suffice for most rocks. 



But that form depending on the mode in which rocks split, 

 the whole impulse should fall on as narrow a line as possible 1 . 

 In practice, however, that cannot be; because the centre of 

 percussion of a wedge cannot easily be made to impinge verti- 

 cally on a surface. But the same object is sufficiently attain- 

 ed by the impulse on a point, through a spheroidal form : as, 

 in a sphere, it is evident that every direction will be, equally, 

 and always, fully, effectual. Hut this being difficult to con- 

 struct, is also unnecessary, while the oblate spheroidal form is 

 even belter; since the curvature of the equatorial portion al- 

 lows of a narrower point of contact, with an equal weight 

 than a sphere would do : the breadth of that contact diminish- 

 ing the power, for obvious reasons." — Vol. ii. pp. 4G'G, 467. 



The chapter on the " Relations of Organic Remains" is per- 

 haps the most extraordinary and intemperate pari of the hook. 

 But to do Dr MacCulloch justice, we coincide much in (he fol- 

 lowing views with which it opens : " The increase of know- 



ledge has given a very different complexion to this subject. 



