Take, for example, cold punched nuts. Those made by Messrs. Hoopes & Townsend, Philadelphia, when taken as specimens of "commercial," as distinguished from merely experimental punching, are of considerable interest in this connection, owing to the entire absence of the conical holes above mentioned. When the holes are punched by machines properly built, with the punch accurately fitted to the die, the effect is that the metal is made to flow around the punch, and thus is made more dense and stronger. That some such action takes place seems probable, from the appearance of the holes in the Hoopes & Townsend nuts, which are straight and almost as smooth as though they were drilled. Therefore I repeat that iron or steel that is not improved by proper punching machinery is not of fit quality to enter into the construction of steam boilers. STRENGTH OF PUNCHED AND DRILLED IRON BARS. HOOPES & TOWNSEND. ----------------+------------------+----------------+----------------+ Thickness of bar|Thickness outside | Punched bars | Drilled bars | in inches. |of hole in inches.|broke in pounds.|broke in pounds.| ----------------+------------------+----------------+----------------+ 3/8 or 0.375 | 3/8 or 0.375 | 31,740 | 28,000 | 3/8 or 0.375 | 3/8 or 0.375 | 31,380 | 26,950 | 5/8 or 0.625 | 1/4 or 0.25 | 18,820 | 18,000 | 5/8 or 0.625 | 1/4 or 0.25 | 18,750 | 17,590 | 5/8 or 0.625 | 3/16 or 0.1875 | 14,590 | 13,230 | 5/8 or 0.625 | 3/16 or 0.1875 | 15,420 | 13,750 | 5/8 or 0.625 | 1/8 or 0.125 | 10,670 | 9,320 | 5/8 or 0.625 | 1/8 or 0.125 | 11,730 | 9,580 | ---------------------------------------------------------------------+ It will be seen from the above that the punched bars had the greatest strength, indicating that punching had the effect of strengthening instead of weakening the metal. These experiments have given results just the reverse of similar experiments made on boiler plates; but the material, such as above experimented upon, is what should be placed in boilers, tough and ductile, and the manner of, and care taken in, punching contribute to these results. It is usual to have the rivet holes one-sixteenth of an inch in diameter larger than the rivets, in order to allow for their expansion when hot; it is evident, however, that the difference between the diameters of the rivet hole and of the rivet should vary with the size of the rivet. The hole in the die is made larger than the punch; for ordinary work the proportion of their respective diameters varies from 1:1.5 to 1:2. As I have before stated, the best plate joint is that in which the strength of the plate and the resistance of the rivet to shearing are equal to each other.