Fig. 3 is the construction commonly made by engine-makers. Its defects are as follows:--From the form of the furrows and ridges where the leather is tied it does not hold on well against a force tending to pull the hose off end-ways; screw-nails are therefore often employed, as at A, to secure the hose on the brass. The points of these nails always protrude more or less into the inside of the joint, and materially impede the current of water. The mouths of the joints are also turned outwards, and form a shoulder, as at B. The intention of this is probably to assist in securing the leather in its place, and to prevent the lapping from slipping. The effects of it are as follows:--First, from the leather being strained over this projection, it becomes liable to be cut by every accidental injury, and very soon cracks and gives way, when a portion must be cut off and a fresh fixing made; second, the leather being stretched over the projection, does not fit the other part of the joint, and must be loose or filled up with pieces of leather, or, as is sometimes done, with brown paper; third, the irregularity of the calibre of the conduit which this shoulder occasions diminishes the performance of the engine. [Illustration: FIG. 4. New Coupling] Fig. 4 is the coupling-joint adopted in Edinburgh. The furrows at the tying place are shallow, but their edges present a powerful obstacle to the slipping of the leather. No screw-nails are employed, nor is there any shoulder, as at B; there is therefore no impediment to or variation in the velocity of the current, as the calibres of the coupling joints and of the hose are so nearly uniform. It will be seen also that as the lapping projects above the leather this latter can never be injured by falls or rubbing on the ground. Another great advantage attending the joints used here is the manner in which their screws are finished. On examining the figure minutely, it will be observed that the male-screw ends in a cylinder of the diameter of the _bottom_ of its thread, consequently of the diameter of the top of the thread of the female-screw. The effect of this is, that, when the screws are brought together, the cylindric portion serves as a guide to the threads, and the most inexperienced person cannot fail to make them catch fair at the first trial. The advantage of this in the circumstances attending fires is obvious. These joints, although requiring three or four turns to close them up, yet as it is only the ring D which requires to be turned, it can easily be done with the hand alone without the use of wrenches. Although, when the whole length of hose has been jointed, it may be as well to send a man with a pair of wrenches to set the joints firm; this, however, is by no means absolutely necessary; if the joints are kept in proper order a man can secure them sufficiently with the hand.