So the number of inches of air area, is 8-20/800 when the drill is propelled upon the rock, and 7-7/100 when returning from it. A full complement of men to work the machines first used, would be, perhaps, thirteen. Mr. Gates, who superintended their operation in the first instance, began with fourteen, but they were reduced to thirteen. The Burleigh machine practically requires feeding, and a full set at work would probably demand fifteen men for their successful operation. The value of these machines has not yet been ascertained. The Committee are of opinion that when a full complement shall have been obtained, so that the workmen can have at all times a full supply upon the frames, that greater progress can be obtained by them than by hand drilling; and after a few months operation, the cost of using them, in comparison with hand labor, can be fairly tested. But as the Brooks, Burleigh and Gates machine has been abandoned, no useful results would be obtained by comparing the expenses within the tunnel during the months of July, August and September, with three corresponding months when hand drilling was carried on, and no satisfactory comparison can be made between the working of the Burleigh machine and hand drilling, until a sufficient number of machines has been introduced into the tunnel to keep the men fully employed. It is to be hoped that machines sufficient to make the test may be soon obtained, and that this desirable information may be made known. The introduction of the first machine into the tunnel, before its capacity, strength, and expense of working had been fully tested, was unfortunate, inasmuch as its use there delayed the progress of the work. The second machine gives such promise of success, that it will be continued in use in the tunnel until a fair test has been made. But should the Burleigh machine prove unsuccessful, and further attempts with machines be attempted, the Committee recommend that their usefulness be tested outside of the tunnel, and meanwhile the excavation with hand drills be resumed. EXPERIMENTS WITH DR. EHRHARDT'S POWDER. The first blast was fired in the tunnel at the East End on the 20th of November, but owing to the presence of charcoal or some other substance in the article, a poisonous gas was evolved which effected the miners disagreeably, and drove them from the work. The subsequent experiments at this point were not satisfactory, and were discontinued, and subsequently resumed at the Central Shaft, where it was used most of the time for a week, varying its composition from time to time. At the close of the week, while preparing for the last blast, a premature explosion took place, resulting in the death of one of the miners, and the injury of three or four others.