OF GUN-METAL.

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of the largest 8580. Shot perfectly spherical and well po-lished were cast for the occasion; none were used that werenot exactly 8290 of the same parts in diameter; the propor-tion between the calibre of the gun and the diameter ofthe snot was in the gun of the smallest calibre as 31 to 30,in that of the mean as 27 to 26, and in that of the largestas 22 to 2i- They were first proved as usual with threerounds, charged each time with 9 Ib. 14 oz. of powder andlaid at 15 0 , to fee how the casting had succeeded; hoflaw appeared on examination, except at the position of thewad between the powder and shot, where there was a smallcavity; but its depth was not measured, as the instrumentfor that purpose had not then been invented (39).

In May 1753, these guns mounted on their carriages,were placed on horizontal platforms, charged with 6f Ib. ofpowder, and directed towards the butt; fifty rounds weresired in the morning in the space of four hours, and an equalnumber in the afternoon, in the fame time. The principalobject being to observe in what manner the guns were ren-dered unserviceable, they were examined at first after everyten rounds; but so soon as the least flaw was perceptible be-tween the muzzle and the part where the shot was lodged,they were examined after every round. The gun of themedium calibre became first unserviceable; some little cavi-ties were formed between the 40 th and 50 th rounds ; theseincreasing in number and size, we observed between the110 th and 120 th rounds several fissures; the muzzle alsobegan to lose its shape, the number and depth of the cavitiesincreased from the 126 th to the 137 th ; and at the 139 th roundthe shot broke to pieces; the firing was then discontinued, asthe muzzle would probably have dropped at the next round.

The next gun damaged was that of the smallest calibre ;some small cavities were perceived in the bore of this piece, be-tween the 50 th and 60 th rounds, and several furrows betweenthe no th and 170 th ; at the 208 th and 214 th the furrows weremuch deeper, and the metal was cracked in several partsentirely through ; these fissures increasing between the 220 thand 230 th rounds, the gun became unserviceable at the 233 rd ,when the shot came out in pieces.

In the gun of the largest calibre, some cavities appearedbetween the 30 th and 40 th rounds, and some furrows at the130 th , which enlarged at the 219 th and 230 th , with severalfissures that penetrated to the exterior surface at the 240 thfound; and as they were still increasing, it was judged un-H 4 necessary