H.—ls

10

boilers are, however, of a special design suitable for installing with any steam plant. Progress in the matter of design is evident in the locomotive and traction types of boiler. One firm is making a corrugated or grooved form of flat plate which is self-supporting and does not require staying. Another firm is dispensing with the stayed construction by forming such plates as were usually flat in the form of cylinders or portions of cylinders. Every stay or other hole in a boiler is a potential source of leakage and consequent corrosion. Such construction should prolong the life of the boiler and facilitate the cleaning of the internal surfaces. Local firms continue to make numbers of boilers for dairy factories, freezing-works, &c. Some of these boilers of the multitubular underfirod type are of large size, 70 and 85 nominal horse-power. During the year replies have been received from some twenty-six firms in Great Britain and America in answer to requests for particulars of the ship and boiler material manufactured by them. This information has now been printed on a sheet in a handy form for reference ; the sheet shows the class of material manufactured, by the different firms, the brands used to distinguish the various classes of plates, &c., the position, of the brands, and any other information of interest which was received. The number of new boilers inspected during the year was 347 ; 162 of these were made in the Dominion and 185 were imported. The following table gives the districts in which the new boilers were installed, their horse-power, and also states whether they were made in the Dominion or imported . —

Inspection of Boilers. The number of boilers inspected this year totals 7,746. This shows an increase of 473 on the number inspected last year. The total number of defects in boilers was 1,592, and of this number 334 were very dangerous. No boiler-explosion occurred during the year, which shows the efficiency of close inspection made by reliable and competent practical men. In October last year it was reported that a fly-wheel of an engine had exploded. The fly-wheel was attached and formed part of a steam-engine plant driven by steam generated by a steam-boiler. The damage was estimated at £13,000. It is not generally realized how dangerous fly-wheels are, and a few remarks concerning them may be useful to steam-users having no technical knowledge of them. The most common cause of fly-wheels bursting is from an increase of speed due to defective governors or valve-gears. The main stresses in a fly-wheel are the result of the centrifugal force of the rim due to the speed at which the fly-wheel revolves. There is no possible way of overcoming these stresses. If the sectional area of the rim is increased to make a stronger rim the centrifugal force is increased likewise, because of the added weight of the rim, and the wheel is no stronger than before. There is a definite speed at which any wheel will explode, however sound, and regardless of the amount of material it contains. This theory is a mathematical truth, and has often, been verified by practice. Cast-iron wheels cast in one piece should not be run at a greater peripheral speed than a mile a minute. For built-up wheels the speed should be much less, and according to the design of the rimjoints; some rim-joints have a strength equal to 25 per cent, only of the solid rim. The stresses in a revolving fly-wheel increase as the square of the specd —that is to say, if the speed attains double the normal speed the stresses will be four times the normal stresses. This rapid increase of stress due to speed is the reason why wheels amply safe at ordinary running-speeds go to pieces without warning and cause such destruction. Fly-wheels are ordinarily made of cast iron, and require to be thoroughly examined periodically for flaws and defects. Steam-users should see that all governor and valve gear is kept in first-class condition, so that the danger of the engine running away is minimized. A fly-wheel merely averages the speed of an engine during one revolution, and prevents violent changes in the rate of motion in that time. The number of revolutions per minute can only

District. Made in Dominion. Number. Horse - power. Impi Number. irted. Total. Horse - Number. Hor8e - power. power. Auckland Auckland North .. Auckland South. .. Canterbury North. . Canterbury South Gisborne Hawke's Bay Marlborough Otago Southland Taranaki Taranaki North Wellington Wellington North Westland M (i 7 20 3 6 14 4 10 10 5 8 29 20 6 130 52 62 201 29i 15 27 12 68 146 44| 431 126 156J 801 22 4 18 12 5 23 2 20 20 ir, 5 30 5 4 239 82 980 34 23 152 5 205 50 35 209 1,068 37 39 36 10 25 32 3 11 37 6 30 30 20 13 59 25 10 369 134 1,042 235 29-J-38 179 17 273 196 252| 1,194 1931 1191 Totals .. 162 1,193* 185 3,158 34-7 4,351|