the opposite direction. By means of this most suitable arrangement of counter currents, all the heat produced by the flame is transferred to the water, and the spent gases escape through the throttle at atmospheric temperature. The pressure of the water current is kept constant by two overflows, 3 and 20, and

the quantity of water passing through the apparatus can be regulated by the stopcock 9. A baffle plate, 14, surrounds the lower end of the tubes to ensure an even distribution of the

water, and in the neck of the apparatus, at 38, several discs with cross slots are arranged to ensure an intimate mixture of the heated water before it reaches the thermometers. Provision is made to collect the water which is formed during the combustion of hydrogen in the gases in the annular space, 34, and to pass it into a measure glass through the tube 35. To prevent radiation, the whole body of the apparatus is enclosed in an air jacket.

As the standard unit of heat generally used is the calorie, i.e., the amount of heat required to raise the temperature of 1 kilogramme (1 litre) of water 1° Cent., this unit has been adopted in Junker's Calorimeter.

If British Thermal Units are required, the result in calories has only to be multiplied by the factor 4 (more correctly 3.9683).

Taking the Reading

When the pointer of the gas meter passes zero, or a whole figure, shift the hot water tube from over the funnel into the measuring glass, and note the temperature of the hot water thermometer at five or six intervals while the glass is being filled. The cold water thermometer will generally remain stationary, and need only be observed once. As soon as the hot water reaches the two-litre mark, turn the gas off, and read the quantity of gas shown by the meter.

The following readings will serve as an example :

A calorie being the quantity of heat required to raise the temperature of 1 litre of water 1° Cent., the experiment will

show the heating value of the gas by means of the following equation :

where H is the calorific value of one cubic foot of gas; W is the quantity in litres of water heated;

T is the difference of the temperature in degrees Centigrade of the inflowing, and of that of the outflowing, water;

G is the quantity in cubic feet of gas burned during the experiment.

W = 2 litres, T = 26.77° 8.77° 18°;

104.65 calories.

A cubic foot of this gas would therefore have a calorific value of 104.65 calories.

It should be observed that this is a 'gross' value, which represents the total heat generated by the flame, including the whole of that of the hydrogen contained in the gas, which in the calorimeter is converted into water, and gives up its latent heat to the circulating water.

It is therefore of importance to ascertain the 'nett' calorific value of the gas used in such processes, which in many cases is 10 per cent. less than the gross value. The calorimeter gives a ready method of determining the difference between these gross and nett values, as we have only to measure the quantity of water condensed in the apparatus and collected in the small measuring glass. For every cubic centimeter of this water, an allowance of 0.6 calorie must be made. As the quantity of water produced is proportionally small, it is advisable to burn a large quantity of gas, say 2 to 3 cubic feet, for these determinations. Supposing 2 cubic feet of gas condensed 53 c.c. of water, we should ascertain the calorific value of the latent heat of the condensed water, thus:

which should be deducted from the gross value, leaving the nett calorific value equal to 88.75 calories per cubic foot.

Professor William Robinson, assisted by Mr. Alfred Hay, B.Sc., made with Junker's calorimeter a series of 130 calorimetric tests with Nottingham gas. The author was present and assisted during one test. All temperatures are given in Centigrade, the heating values in calories.

During the first and fourth day, samples of the gas were analysed by Professor Frank Clowes, D.Sc. By calculation from analysis the heating value of the gas was ascertained to be 164 calories gross, which fairly agrees with the above results.

Professor Robinson has also made some careful tests of Dowson gas, of which the following table gives the results :

The gas was taken from the main supply to the engine during a run. Recently, by using a Shaw gas governor, much less variation is shown in the water outlet temperature, and the calorific value of the gas is thus exactly determined during the gas engine test.

CHAPTER XXXIII.

DOWSON GAS

THE economical conditions of obtaining power from coal or coke are daily becoming more understood, and to obtain an indicated horse-power hour from one pound of coal is a great achievement; but that it has been done is indisputable, the credit for which must to a great extent be given to Mr. J. Emerson Dowson, who has designed a plant for, the manufacture of producer gas, very suitable for a gas engine, with which the above result has been obtained. Dowson gas is made by forcing a mixture of steam and air through a mass of red-hot fuel, when not only is the steam decomposed into its constituent gases, oxygen and hydrogen, but a sufficiently high temperature is maintained in the generator to carry on the process continuously and make the gas as it is required by the gas engine.

A complete and compact set of his plant is shown at figs. 191 and 192, capable of generating sufficient gas for 80 E.H.P.

It consists of the boiler A, which is fitted with superheating tubes, B air injector, C gas generator, D feeding hopper, E fire bars, F gas cooler, G waste pipe, H hydraulic box, I overflow, J sawdust scrubber, K coke scrubber inside tank of gas holder, L gas holder, M outlet from gas holder to engine, N N ashpit for generator, O automatic regulator to govern production of gas.

The process is as follows. A current of superheated steam passes continuously, by a pipe from the top of boiler, through the injector, which consists of a nozzle inserted in the mouth of a conical tube, open to the atmosphere.

The pressure of steam varies from 30 to 60 lbs. per square inch above the atmospheric pressure, according to the size of the apparatus used and the quantity of gas required. This pressure forces the mixture of steam and air upwards, through the incandescent fire in the generator. The steam is decomposed in presence of the incandescent carbon, and the hydrogen, being free, passes off. The oxygen from the steam, as well as that from the air, combines in the first instance with the carbon of the fuel to form carbonic acid. As this rises through