48

MODERN STEAM PRACTICE.

balance is restored, thus keeping up constant low pressure, regu-lated at pleasure by the thumb-screw pressing down or releasingthe piston and the valve. One of these valves can be fitted to themain steam-pipe, or a separate one for each cylinder when required.

THE DETERIORATION OF LAND BOILERS.

After a time the plates of all boilers deteriorate, the iron becomesbrittle, and although the plates have a sound-looking exterior, with-out the slightest Symptoms of corrosion, yet such a boiler shouldnot be worked beyond a certain number of years, and certainly notat so high pressure as it was originally designed for; in fact, thesteam pressure should decrease year by year, so as to work it withany degree of safety. It must be understood, however, that unlessa new boiler is properly managed, it is quite as unsafe as a mucholder one well managed. To determine the number of years aboiler ought to last, with fair treatment, we must have recourse toexperiment. When it is thought a boiler has done enough dutytest it to destruction. Such experiments are very easily carried out,and it is the interest of steam users to do so, that correct data maybe arrived at by a careful experimentalist.

We place before our readers the results of a series of experiments,testing two boilers to destruction, instituted by Mr. Peter Carmichael, 1and which forms a useful contribution on the subject of steam-boilers. The boilers were cylindrical, with double flues, and wereused at the Dens Works, Dundee, for nineteen years. They wereprecisely alike, and of the following dimensions:—Length, 25 feet;diameter, 7 feet; diameter of furnaces and end flues, 2 feet 9 inches;diameter of back end of flues, 2 feet 6 inches. The shell was madeof inch “ Glasgow best iron;” the flues of Glasgow best scrap iron,i/z inch thick, the end plates being T V inch in thickness. The boilerswere kept in work until the beginning of November, 1869, when itwas resolved to take one out, and test it to destruction by waterpressure. In the case of the above boilers the pressure has neverbeeil so great as 60 Ibs. , and as reported they were not wasted,having always been kept in good repair, and have stood the peri-odical water test of 60 lbs.; therefore we may presume they couldhave been worked for a year or two longer. The fact of the irongetting hard and brittle after being in use for a length of time had

1 See Trans. Inst. Engineers and Shipbuilders in Scotland \ vol. xiii.