G.— ENGINEERING. 163 



Taking all these factors into consideration, and assuming the popula- 

 tion at the end of the next twenty-five years will have grown to 50 millions, 

 and that the methods and extent of distribution have advanced and 

 developed — coupled with a sensible reduction in the cost of appliances 

 resulting from scientific and commercial improvement and greatly increased 

 scale of manufacture — the total output for domestic requirements, 

 including residential premises, shops, offices, and public places, may bo 

 estimated at not less than 20,000,000,000 units, with a maximum load of 

 8,000,000 kilowatts. 



An interesting speculation may be made and some support afforded 

 to the reasonableness of this estimate from the following figures : — 



In 1924 the amount of raw coal used for domestic purposes was given 

 in the annual report of the Secretary for Mines as 34,280,000 tons, being 

 19 per cent, of the total consumption of coal in Great Britain. 



Observations by the Fuel Research Board have assessed the average 

 thermal efficiency of open coal fires, allowing both for radiation and 

 convection of heat, at 22i per cent, of the total heat contained in the coal 

 consumed. If we assume that the average thermal value of the coal per lb. 

 is 11,000 B.Th.U., an average which is probably lower than the real 

 figure, this tonnage is equivalent to 190,048 x 10' B.Th.U. 



The gas returns published by the Board of Trade show that the average 

 increase in the output between 1913 and 1924 of all the gas undertakings 

 in the United Kingdom was 2.2 per cent, per annum, based on the 1913 

 returns, and the annual increase expressed in therms (assuming an average 

 calorific value of 500 B.Th.U. per cubic foot of gas) was 22.7 x 10' over 

 that period. 



The electricity returns record an average rate of increase in the sales 

 of electricity during the same period of 25.8 per cent, per annum, being 

 a yearly increase of 340,000,000 units, equivalent to 11.6 xlO" therms per 

 annum. Now it has been stated by the Fuel Research Board (Technical 

 Paper No. 12) that the radiation efficiency of modern gas fires is generally 

 from 55 per cent, to 60 per cent., to which must be added about 10 per cent, 

 for convection, as is usually claimed. The radiation efficiency of electric 

 heaters ' varies according to type, but reaches as much as 70 per cent. . . . 

 Whatever the radiation efiiciency, however, the total efficiency is 100 per 

 cent., the balance appearing as direct convection.' 



In future years, through the public demand for effective means of 

 preventing atmospheric pollution and reducing the smoke nuisance, for 

 increased cleanliness of buildings and from other considerations of public 

 health, we can imagine most of the coal now consumed in open grates and 

 kitchen ranges to be replaced by gas or by electrical apparatus. Without 

 making any allowance for increased population, it would be fair to assume 

 that this substituted means of heating at the consumer's premises could 

 be subdivided approximately in the ratio of the recorded increased annual 

 sales of gas and electricity, paying due regard to the efficiency of utilisation 

 in each case. On this basis consumers' heating requirements would be 

 provided as to 22.7 xO.7 =15.9 parts by gas and as to 11.6 xl =11.6 parts 

 by electricity. The resultant annual sales of gas and electricity merely 

 for the purposes of displacing the above-mentioned tonnage of domestic 

 coal would then be : — 



m2 



