A late number of the London Mechanics' Magazine contains an article on the above important question, by W. G. Tosh, from a paper read by him before the Institution of Mechanical Engineers at Manchester, England. He constructed small vertical boilers of equal dimensions, and placed in the center of each a single tube, two inches in diameter; and of No. 14 wire gage thickness. A gas flame was applied to each tube—iron, brass, and copper —successively, during a certain period of time, which was equivalent to the same quantity of fuel consumed in each case. The experiments were first conducted during the day, then at night, at times when there was little probability of a change of pressure in gas pipes. Eight of these were made with the boilers, and the quantity of water evaporated was measured by the number of inches it was lowered in a boiler by each experiment. The result was in favor of the greater evaporating power of the brass over the iron tubes, in the proportion of 125 to 100 ; that is, two pounds or two tuns of coal, or other fuel, will, with the use of brass tubes in a boiler, evaporate twenty-five per cent more water than iron tubes with tho same quantity of fuel, under precisely the same circumstances. In the samo proportion that brass surpassed iron in evaporative power, copper was found to surpass brass. The evaporative powers, relatively, of the three metals in tubes for steam boilers, he found were as follows: Iron, 100 ; brass, 125; copper, 156. The experiments of Mr. Tosh were subjected to a searching criticism by the ongineers of the Institution, and strong doubts were expressed as to their correctness. We give the results, in substance, as wo find them, and enjoin some of our correspondents to make similar experiments, because it is a question of vast importance. If it be true that copper tubes in boilers will evaporate fifty per cent more water than iron tubes, no other kind of tubes should be employed, and no steam boiler should be constructed without copper tubes. In our opinion, too high an evaporative value was obtained both for brass and copper over iron; but this is a question which experiment alone can settle, and the sooner this is done correctly, so much the better for mechanical science. It takes the pressure of 150,000 lbs. to punch an inch hole in an iron plate one inch thick.