All About Engines

106 All About Engines efficiency would yield him; but the man who burns 1,000 tons in the same period is letting £3,000 a year slip through his fingers—or run down the drains, or into the canal or into some other place from which it is irrecoverable. The second man is often the servant of a public company, with inquisitive share- holders demanding higher dividends ; or of a muni- cipality with ratepayers who want long penny stages or low electric light bills. So, for his own sake, directly or indirectly, he demands to be provided with some of the boilers and accessories which have already been described and with engines having all sorts of refinements which reduce, each in its measure, the consumption of steam. For nearly a hundred years after Watt’s inven- tions the chief improvements in steam engines lay in the methods of manufacture—in the gradual re- placement of the individual and variable skill of the workman by the automatic accuracy of the ma- chine tool. Then engineers turned their attention to improving the invention itself. Stronger and more reliable materials, combined with increasing accuracy of workmanship, have enabled higher and higher pressures to be used ; so that the 5 lb. or 10 lb. with which Watt had to be content in his day has been raised to 200 lb. or 250 lb. One problem, therefore, has been the construction of steam-tight joints which shall resist such pressures and, where superheated steam is used, a temperature of 6oo° Fahr, or 700° Fahr. Newcomen made his pistons steam-tight by means