2 Dr. Mac Culloch on Mineralogical Hammers. 



their water, or from the ordinary eflFects of exposure. In such 

 cases it is necessary to procure specimens in succession from 

 the same point ; an attempt, in which ordinary means will often 

 fail, from the gradual loss of the protuberances or an2:les on 

 which alone an impression can be made by a moderate or ordi- 

 nary force. It is also often desirable to obtain a cross fracture 

 of some of the schistose rocks, as in micaceous schist, for the 

 purpose of displaying the contortions. This, from the greater 

 facility with which the laminse yield to a moderate force ac- 

 cording to their direction, can rarely be eflected by any ordi- 

 nary hammer ; requiring a greater and more concentrated im- 

 pulse, and often, indeed, demanding the very sudden effort com- 

 municated by gunpowder. 



The weight of a hammer required to produce such effects, if of 

 the ordinary construction, is a serious inconvenience to a geolo- 

 gist ; who must, in many cases, necessarily examine the ground 

 which he is investigating, on foot, and who is also not unfre- 

 quently incumbered with specimens. Nor will mere weight 

 answer the purpose, as a very slight consideration of the laws 

 which regulate the communication of motion will show. Having 

 at first suffered much inconvenience from the use of hammers of 

 the common construction, the geological readers of the Quarterly 

 Journal will not be sorry to know the expedients which I adopted 

 to diminish it ; and, to render the form which I have used for 

 some years past more intelligible, I have accompanied this com- 

 munication with explanatory sketches. 



That I may not incumber a plain practical question with ma- 

 thematical considerations, I shall only here remark, that although 

 the momentum of a body is compounded of the weight and 

 velocity, and that the same absolute quantity of motion may be 

 communicated under vai'ying relative proportions of these two 

 elements, the disintegration of bodies is regulated by other 

 rules, and a diminution of velocity cannot be compensated in 

 this case by an addition of weight. It is by an increase of the 

 impulse that the cohesion of bodies is overcome : a great 

 weight causes the body to move in one mass ; a great velocity 

 strikes off a fragment, or breaks the whole to atoms. Illus- 



