The Society of Arts have published, in their thirty-first volume of Transactions, descriptions of two fire-escapes or elevators. They consist of a number of bars jointed together in pairs, by a pin in the middle of each like a pair of shears. To the upper ends of each of these the lower ends of a second pair are jointed, and to these a third pair, and so on of five pairs. The whole assemblage will therefore consist of similar parallelograms placed one upon the other. Now, by forcing the lowest ends of the levers to approach towards each other, the parallelograms are caused to elongate in the vertical direction, and raise trp a platform to the required height. The machine, when elevated, forms a lofty tower, within which a regular range of ladders are placed in the manner of a staircase. The contrivance is ingenious, but by no means applicable to the purpose, from its complexity. One of these machines is composed of 24 levers, and 8 ladders, besides the smaller parts. The other machine has 40 levers and 8 ladders within it.
Of the other kind of fire-escapes, which are to be fixed from the window, the most simple is a rope-ladder, with wooden rails for the steps ; but, unless a post with a hook is fixed below in the street, to attach the lower end of the ladder to it, and strain the ropes tight, it is extremely difficult to descend.
Another species i3 called the sling fire-escape. This consists of a rope, to which the person fastens himself by a girdle, and throwing himself out of a window, is lowered slowly down, the rope having some contrivance to cause a friction or resistance, which will prevent any acceleration in the motion. The simplest of these has a long rope provided with two straps or belts, one to buckle round the waist of the person who is to descend, and the other to pass under him, so that he sits as in a swing when suspended by the rope, which is rather more than twice as long as the height of the window from the ground. The rope is made to pass through a double eye or iron ring, suspended from a hook fixed over the window ; then the other end of the rope is brought down to a piece of wood called the regulator, which is attached to the girdle strap, that the person sears. This piece of wood has three holes in it, and two deep notches, into which the rope is woven, and will thereby have so much friction in passing, as to make it slip through regularly, and quite at the command of the descending person who is to hold the rope in his hand; and by letting it slip more or less, he can easily regulate his descent. This plan was proposed by Mr Forster.
Another machine, invented by Mr Maseres, had the same arrangement, except that the rope, instead of pass ing through the iron ring or eye above mentioned, i3 wrapped three or four times round a small cylinder, made with a proper spiral groove, and fixed in an iron shank like the strap of a pulley, but rivetted fast, so that it can not turn tbund. By means of a hook in this shank, the
cylinder can be suspended from the hook which is fixed over the window. The groove in this cylinder causes so much friction in the passage of the rope, that the person who is suspended has an equal command as in the other method ; hut, without passing the rope through the notches in the piece of wood, he can command it when he holds the other end of the rope in his hand, and lets it slip more or less at pleasure.
Another machine, which was exhibited in London, was contained beneath a stool, to stand by the bed-side. On an alarm, this stool could be instantly fixed to the window, by hooking two of its legs over the sill of the window, in the same manner as the boards used by a painter to sup port himself whilst he paints the outside of a window. Beneath the stool was au axle, upon which the rope was wrapped two or three times ; a small wheel was fixed on the end of the axle, and a gripe inclosed this, with a spring to cause sufficient friction to retard the excessive motion of the wheel and axle. The spring was regulated by a screw, to bear upon the wheel with any required force, in proportion to the weight of the person who was to de scend. A strap was placed at each end of the rope, so that when one had descended, the girdle-straps at the opposite end of the rope would be ready for another person to come down.
Mr Salmon has contrived another machine, which ap pears superior to any of these : It is a large pulley, placed in an iron strap, by which it can be hung up over the window. It is made with a deep angular groove, so that the rope which passes over it cannot slip ; and, to render this more secure, the groove has several sharp phis fixed in it : the rope has a girdlestrap at one end, and a sufficient counterweight at the other, to make the rope apply so firmly to the pulley that it cannot slip, and also to draw up the strap the moment the person who has descended has engirded it. The pulley has a toothed wheel fixed against it, which acts in the pallets of an anchor-escapement, fix ed on an axis, placed in the same iron frame or sling, above the axis of the pulley. A short pendulum, with a heavy bob, is fixed on the end of the arbor of the anchor or pallets. By this contrivance the pulley is regulated, as the escapement will not suffer it to move with any more than the intended velocity ; and if the pallets of the anchor are formed of a proper shape, as will be described in the article DonoLoGy, it will make very little difference if the weight which descends is a small or a great one. The escapement-wheel is attached to the pulley by a ratchet wheel and click, which admits the counterweight to draw up the strap-girdle, without acting on the escapement, as soon as the person who has descended quits it ; and the machine is then ready to let down another person.