The subject of explosive compounds for engineering purposes, which has been discussed in several late numbers of our paper, has attracted considerable notice, and though the articles in question have been valuable, they fail to give that precise information which practical men desire, especially in this country, where energy and enterprise are developed in the highest degree. Mining men wish to know what, explosive will do the most execution, considering safety and expense, which includes " time " as well as actual outlay of capital. For example, a railway may have a tunnel to complete before the road, previously constructed, ca*1 be made available. At the Hoosac tunnel it is estimated tltat, with nitro-glycerin, the opening can be made from one to two years earlier than it can be done with gunpowder. We are also informed that the Union Pacific Railway has used large quantities of nitro-glycerin to hurry the completion of a tunnel, and in order to get even a small quantity, the contr'actors purchased a car, and sent with it, at considerable expense, a messenger to hasten it forward. These are practical evidences in regard to nitro-glycerin. But, then, we must not omit to consider the efficiency and relative safety of other modern explosives. In California, dynamite, or giant powder, has been introduced into over 700 mines. In Pennsylvania, the oriental powder has been considerably pushed, with some degree of success. Periodically, the world has thrust upon it, some new development in the useful arts, and, at the present, we have a variety of explosive agencies, forcibly recommended by inventors, each claiming superiority for their respective products, and each claiming positive safety. Before the questions are satisfactorily solved, commercially considered, there will be some loss of life, and we cannot do more than to hope that the loss will be small. Colonel Shaffner, whose letter appears in another column, gives no information relative to the explosive point of the substances enumerated, when produced by concussion. Gunpowder will explode at 600 Fah., Horsley's powder (called in America, Ehrhardt powder), at 430, gun-cotton from 350, Schultz's sporting powder 385, nitro-glycerin 360, and percussion cap fulminate 340 Fah. These respective degrees of explosion mean, that when each is put in a vessel or room, they will explode when the temperature given is attained. But who can tell the exploding point under conditions of percussion—under a tap of the hammer, whether of metal, stone, or wood ? Each explosive may have thrown around it all the precautions of safety, but, after all, mining men will have the article that will best subserve their interests, and, thus considering the subject, we can only indulge the hope that a proper regard for human life will not be overlooked by manufacturers and consumers, and that they will exercise those precautions which will lessen hazard and secure success to the greatest number. Among the most hazardous of all the explosives claiming the attention of engineers, nitro-glycerin undoubtedly stands at the head, and its efficiency over that of its derivative, dynamite, is not siifficient, in our opinion, to compensate for the hazard involved in its use and transportation. We feel it our duty to express a decided preference for dynamite, where a very powerful explosive is required. The frightful accidents which have occurred from the use of nitro-glycerin, have no parallel in the history of any other explosive compound, and when we take into consideration the difficulty in enforcing care in its handling and packing, we do not hesitate to assert our opinion that its indiscriminate use should be prohibited by statute. We see that some foreign papers take opposite ground in regard to safety attending the manufacture of the Schultz sporting powder, from that taken by the author of the series of articles which have been called in question. In order that our readers may judge for themselves, we publish, in another column, a description of the process by which it is made. The subject of explosive compounds is an important one, and worthy of the fullest discussion.