is pumped to a pressure of 80 lbs. and from which the cylinders are charged.

Dowson's Starter No. 7

When Dowson gas is used, the steam generated in the small boiler used in the manufacture of his gas is utilised for starting the engine. Usually the steam pressure is about 60 lbs. per square inch. To start the engine, the crank is placed over the dead centre on the firing stroke, and steam is admitted until the piston has nearly reached the end of the out-stroke. Where a number of impulses is necessary a special cam is arranged to admit steam for several alternate revolutions.

CHAPTER XV

THE BUNSEN BURNER

FIG. 139 is a part sectional elevation, showing a neat form of Bunsen burner used for heating the ignition tube by Messrs.

Crossley Brothers and other makers of gas engines. The gas

issues from a small jet at the base of the gas supply, and mixes with air which is drawn in through three openings in the tube; it then burns at the end of the tube with a non-luminous flame. The existence of this flame in its ordinary condition depends upon two main causes; first, upon the fact that in the immediate neighbourhood of a jet of gas issuing from a small orifice there is a reduction of pressure; and, second, upon the relation between the velocity at which the gases pass up the tube and the rate of propagation of combustion in the mixture of air and coal gas downwards. Upon the first of these causes depends the entrance of air into the air-holes' of the tube, and upon the second depends the continuance of the flame in its position upon the end of the tube. The consumption of gas equals 3 to 4 cubic feet per hour. The method of adjusting the gas supply is simple and effective.

TEMPERATURE IN DIFFERENT PARTS OF A BUNSEN FLAME (GAS CONSUMPTION 6 CUBIC FEET PER HOUR) Professor Vivian B. Lewes

THE method of firing the charge in the combustion chamber by means of an incandescent tube is now common to all makes of gas engines, the slide with its open flame having been abandoned. The compression gas engine by Mr. James Atkinson in 1879 was the first to work with a hot tube ignition practically the same as that used at the present day. An igniting

tube of wrought iron or ordinary gas pipe, open at its lower end to a hole drilled through the cylinder wall, was used in his Differential engine exhibited at the Inventions Exhibition in 1885. Early in 1881 Mr. William Watson patented various methods of igniting gases by applying heat externally to some part of the vessel, chamber, or tube in which the gases to be exploded are contained; in fact, the timing valve and bulb as we now have it is only a slight modification of Watson's method. Yet it was not until 1888 that the makers of the 'Otto' engine finally decided to substitute the tube ignition for the open flame and slide, and it is a disputed point even now amongst the best authorities on the gas engine whether the tube should be used with its opening always free to the combustion chamber, or whether a timing valve should be used, closing the opening during a certain portion of the stroke. The author holds the opinion very strongly that this valve neither conduces to economy nor power, but would point out that automatic tube ignition needs good governing arrangements, as it detects any defects in the governor very quickly, and a timing valve may hide any defects in this direction.

The Robey Automatic Tube Igniter

Fig. 140 is a sectional elevation, and fig. 141 an end view of an angular automatic tube igniter. A is the combustion chamber, B the igniter passage, C the Bunsen burner. The advantage gained by this arrangement is that the flame naturally surrounds the tube. The igniter passage is shortened, and ignition can be timed without disconnecting any part, by simply turning the Bunsen burner on the pivot D.

The Fielding Automatic Tube Igniter

Fig. 142 is a sectional elevation of a vertical automatic tube igniter introduced by Mr. J. Fielding in 1890.

The special feature in this arrangement is that the Bunsen

burner used is arranged so that the mixture of gas and air issues laterally towards the centre from an annular groove surrounding the tube, thus impinging directly upon it. Air is supplied in suitable proportions below and above the flame to ensure

FIG. 142.-AUTOMATIC TUBE IGNITER--FIELDING

perfect combustion; the usual chimney lined with non-conducting material is provided above the burner, which is arranged so that falling flakes or scale from the tube may drop clear without danger of clogging up the gas or air passages.

CHAPTER XVII

IGNITION TUBES

THE life of an ignition tube depends upon the material it is made of, but varies greatly according to the usage it receives. This life is shortened by intense heating, with sudden changes in pressure and temperature produced by the inrush of comparatively cool gases, whereas gradual changes tend to longevity. It is found that wrought-iron tubes deteriorate rapidly owing to high pressure within the cylinder at the moment of ignition. The thin tube, raised to white heat, is only exposed to the