the company should, upon the laying down of any main-pipe or other pipe in any street, fix, at the time of laying down such pipe, a proper and sufficient fire-plug in each such street, and should deliver the key or keys of such fire-plug to the persons having the care of the engine-house in or near to the said street, and cause another key to be hung up in the watch-house in or near to the said street. By sect. 87, pipes were to be eighteen inches beneath the surface of the soil. By the 89th section, the mains were at all times to be kept charged with water. The defendants derived no profit from the maintenance of the plugs distinct from the general profits of the whole business, but such maintenance was one of the conditions under which they were permitted to exercise the privileges given by the Act. The main-pipe opposite the house of the plaintiff was more than eighteen inches below the surface. The fire-plug was constructed according to the best known system, and the materials of it were at the time of the accident sound and in good order.