Dr. Mac Culloch on Mineralogkal Hammers. 9 



In mineralogical journeys this is particularly convenient, as a 

 trimming hammer soon wears out, and the collector must then 

 carry unnecessary weight, or perhaps fail entirely in procuring 

 convenient and well-shaped specimens. In a collection of rocks, 

 where the number and weight, and the room occupied, form so 

 serious an inconvenience, the regular shape of a specimen is an 

 object of no slight moment. 



There is an additional advantage in this form of the trimming 

 hammer, arising from the rectangular shapes of the edges, which 

 renders them more durable than those of the axe-shaped ham- 

 mer. I need scarcely say that they must be regular prisms, 

 that the eye may see the edge which strikes, and that they must 

 be entirely made of steel. 



With respect to the weights of trimming-hammers, they must 

 be proportioned chiefly to the weight of the specimen to be 

 broken or shaped, or to the size of the fragments which it will be 

 requisite to detach. They must also bear some relation to the 

 fragility of the specimen ; the most brittle requiring the lightest 

 hammers. It is not possible to give any exact rules on this 

 subject, but the general principle has already been stated suffi- 

 ciently to show that it is only by the velocity of a small weight, 

 or by the impulse, that fragments can be detached from any 

 desired place without disturbing other parts of the specimen. 

 The mineralogist should be provided with different weights, 

 from a drachm to two ounces, and upwards; and his own ex- 

 perience will very shortly direct him to that which will produce 

 the desired effect on any specimen or substance under trial. 

 To facilitate the labours of the artist, I have thought it better 

 to insert a scale of dimensions than of weights, for a set of such 

 hammers, and they are as follows : — 



