APPENDIX. 467 



But that form depending on the mode in which rocks 

 split, the whole impulse should fall on as narrow a line 

 as possible. In practice, however, that cannot be ; 

 because the centre of percussion of a wedge cannot 

 easily be made to impinge vertically on a surface. But 

 the same object is sufficiently attained 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. But this being difficult to 

 construct, is also unnecessary, while the oblate sphe- 

 roidal form is even better ; since the curvature of the 

 equatorial portion allows of a narrower point of con- 

 tact, with an equal weight, than a sphere would do : 

 the breadth of that contact diminishing the power, for 

 obvious reasons. With such a hammer, while every 

 blow is effectual, it may be used blindfold, when the 

 wedge hammer fails oftener than it succeeds ; while, 

 admitting a round handle, and demanding no grasp, 

 its head also forming no lever against the hand, as the 

 long wedge does, it requires no muscular exertion in 

 the fore-arm, and cannot strain the wrist by missing. 

 Its duration too is almost eternal ; since, if too hard, 

 the steel cannot chip off, while, if too soft, any former 

 indentation is filled by a second blow. If, thus saving 

 labour, our labourers have not adopted it, after fifteen 

 years, it is perhaps explained by the indignation of 

 the Welsh quarryman at the presumption which could 

 propose a better hammer at this age of the world. 

 And the best form for trimming hammers, is that of 

 a tripled cube of steel, the weights ranging from three 

 drachms to an ounce ; as the smaller fragments de- 

 mand the least weights, according to the foregone re- 

 marks on impulse and momentum. While also such 

 a hammer possesses eight cutting edges, by reversing 

 the handle, it is quickly repaired by grinding the two 



H H 2 



