OF GUN-METAL.

I 10

30. We gather from these experiments;

1. That tin is the principal ingredient towards ren-dering the bronze hard.

2- That the holes made in the metals A, B, C, D,E, F, composed only of copper and tin are nearly in thereciprocal ratio of the quantities of tin.

3. That by adding a quantity of brass, the propor-tion of the ingredients may be diversified, yet retain thesame hardness, as appears from G and L.

4. That the metals C, D, whose component partsare within the limits assigned (27) may serve as a ruleto judge whether the other metals have the requisitehardness for artillery. For example, to compare thehardness of a bronze Z with that of C or D ; let thepenetration of a punch falling on a piece of fine Englissitin of a determined quality —I, then find the penetrationof the fame punch into the bronze proposed, and ex-press it by Z; -7^ will be the proportion between these

penetrations. But by the experiments in the precedingparagraph — is the proportion between the penetra-tions into the tin and the bronze D. If then-^,=—,

Z 40

the proposed bronze Z will be as hard as the given

r

bronze DZ<4o'

If — 7 — Z will be harder than D, and ifZ 40

D will be harder than Z.

31. There are different methods of finding the proportionof tin in any bronze, but they give at best but an approxi-mation.

In the first place, the composition of a bronze may bedetermined by finding experimentally its hardness andtenacity, and comparing them with the hardness (29)and tenacity (Instit. Phyf. Mic. 64.) of a bronze givenas a rule ; if they correspond in these two properties,they may be said to be of the same quality ; but if onebe equally tenacious, but less hard than the other, or

equally