safety-valve in an iron box, and so put it beyond the control of the engine-man.

The annexed figure represents an inaccessible safety valve, calculated to answer all the purposes for which it is intended, namely, the preservation of those employed in the neighbourhood of the boiler, and economy in the use of steam.

In this, as in the preceding diagram, A represents the boiler, and B the safety-valve, furnished with a small upright staff, on which slide the additional weights CCC. The whole is inclosed in a box D, pierced with holes to allow the steam to escape after it has raised the valve B.

Should high-pressure steam be wanted, it is necessary only to increase the number of weights, and the desired effect is produced; or if, on the contrary, steam of the usual atmospheric pressure be wanted, the whole of the weights are taken off.

The safety-valve invented by the Chevalier Edelcrantz, has nearly the same properties as that employed by Mr. Woolf. It consists of a small brass cylinder which is fixed on the boiler, and fitted with a piston made to descend with its own weight when raised by the pressure of the steam.

The lower part of the cylinder being made to communicate with the boiler; the upper part is closed by a small cover screwed on to it, and perforated with a hole, through which the piston-rod passes freely, which serves the double purpose of keeping the piston perpendicular, and preventing it being blown out. The sides of the cylinder are pierced with a number of small holes, placed in succession at a short distance above each other, so that the open space for the steam to escape, increases with the height of the valve, and is ultimately enlarged so as to prevent any danger of explosion. The piston-rod is also furnished with a number of weights, fitting loosely on a small shoulder, similar to those employed in the common hydrometer; and these may be removed or increased at pleasure.

Another advantage likely to result from the adoption of this safety-valve is, the facility with which it may be employed to regulate the fire of the steam-engine furnace to the intensity of the elastic vapour required. This may readily be effected by a register pressing on the top of the safety-piston, and connected with the apertures for the admission of air, which, by increasing or decreasing the supply of oxygen, will have a proportionate result on the steam generated in the boiler, and consequently effect a considerable saving in the expenditure of fuel.

Another safety-valve, opening internally, has, we believe, also been added by Messrs. Boulton and

Watt. This is of great utility, more particularly in large engines, as it prevents the sides of the boiler being crushed in by the sudden introduction of water, or any artificial condensation that may take place from reducing the heat of the boiler-head.

CHAPTER VI.

Savery's Engine improved by Pontifex.-Atmospheric Engine.-Single-acting Engine, by Boulton and Watt.—Murray and Wood's Engine. High-pressure Engine.-Woolf's Double-cylin der Expansion Engine.-Maudslay's Portable Engine.-Masterman's Rotatory Engine.-Smokeconsuming Furnaces.

THE Engine invented by Savery, and improved by Pontifex, possesses considerable advantages over the Marquis of Worcester's apparatus, and it is probable that the extreme simplicity of this engine will, when better known, bring it into more general use. With this view we have selected it as the subject of our first plate, in preference to the original engine, the principle of which has been already very fully explained. The apparatus we are about to describe, has lately been erected at the City Gas Works.

Plate I. Fig. 1. and 2. represent front and side elevations of the cylinders, and connecting apparatus. Fig. 3. Back view, with section through the cistern and buckets.

L

Fig. 4. and 5. Vertical and horizontal sections; the latter commencing at the dotted line aa, Fig. 1. Fig. 6. and 7. Side and end view of the waggon boiler.

b.b. Fig. 1. Two steam cylinders connected by cross tubes at cc, in each of which a vacuum is alternately formed by the condensation of elastic vapour, conducted from the boiler by the benttube d, and admitted to the steam-cylinders by means of the sliding-valve e.

f.f. Fig. 4. Two tubes perforated with small holes for the admission of steam and injection water, the latter of which is distributed by falling on the strap g.

h. The suction-pipe proceeding to the bottom of the well, which in no case ought to exceed from twenty-eight to thirty feet in depth; so that a vacuum being formed in the copper vessels bb, the water will be raised by the pressure of the atmosphere, and passing up the tube h will take the place of the elastic vapour.

i. i. Two valves placed at the upper end of the suction-pipe h, which allow of the upper passage of the water from the well, but prevent its

return.

j.j. Two similar valves opening into the airvessel k, to which is attached the nozzle l, serving to convey the water from the copper vessels to any required point.

m. The injection tube, furnished with a valve