CHAMBERS'S INFORMATION FOR THE PEOPLE. 



the steam in the boiler will be able to lift the 

 valve. For most stationary boilers, a weight 

 which can be moved along the lever to any 

 desired position is used instead of the spring. 



Fig. 4. 



In marine boilers, the lever is frequently dispensed 

 with altogether, and weights act directly on the 

 valve. It is also usual, and in steamers com- 

 pulsory by law, to put two safety-valves on each 

 boiler, one adjustable at will by the engineer, and 

 the other (called the ' government valve ') inclosed 

 in a case to which access can only be obtained 

 by means of a key in the possession of the captain. 

 The danger of explosion through overloading the 

 valves is thus obviated. 



Although the safety-valve regulates the maxi- 

 mum extent to which the pressure in the boiler 

 can go, it does not indicate its amount or degree. 

 For this purpose, a ' steam-gauge ' is used. This 

 generally consists of a small metallic tube (having 

 an elliptical cross section), curved in the form of 

 the letter C, one end of which is fixed, and the 

 other connected by wheelwork or links to an index 

 like a clock-hand. Steam is allowed entrance to 

 the tube at the fixed end, and the higher its 

 pressure, the more the tube tends to straighten 

 itself. The exact amount of this tendency for the 

 different pressures is ascertained, and the dial- 

 plate graduated accordingly. The whole gauge is 

 inclosed in a cylindrical brass box with a glass 

 front. 



As serious consequences may result from trie 

 water becoming too low in the boiler, and much 

 inconvenience from its being too high, it is neces- 

 sary always to know its exact level. This is 

 ascertained by two methods namely, by gauge- 

 cocks, and by a water-gauge. The former are 

 three cocks communicating with the boiler, one 

 just at the right water-level, and the others 

 respectively at the highest and lowest levels that 

 can be safely used. By opening these cocks, 

 therefore, the engine-driver can see 

 (according as steam or water is 

 blown out) the water-level, within 

 certain limits. The water-gauge, 

 however, tells its own tale with- 

 out compelling any action on the 

 part of the driver. It is shewn 

 c ,u in fig. 5, where aa is part of the 

 lp front of the boiler, and bb the 

 level of the water. A pipe, d, com- 

 municates with the steam-space of 

 the boiler, and another, e, with the 

 Fig. 5. water-space ; these are connected 

 by a glass tube c, and stop-cocks, g 

 and/, are so placed as to cut off the connection in 

 the event of the tube breaking. When both cocks 



422 



are open, the level of the water in c is obviously 

 the same as that at bb, so that the engineer can 

 see at a glance how the water in his boiler 

 stands. 



Many contrivances have been devised to insure 

 that the boiler shall always have a sufficient supply 

 of water. Some of these are of the nature of a 

 ' self-acting feed,' but the greater part are simply 

 alarums or whistles, so arranged as to make a 

 great noise, which the most careless attendant 

 cannot help hearing, directly the water-level 

 descends below a certain point. 



Patents have been repeatedly taken out for 

 inventions intended to do away with hand-firing,, 

 so that the stoker should have nothing to do but 

 to fill a hopper in front of the boiler, while the 

 actual feeding of the furnace was done by ma- 

 chinery. Some of these inventions have been 

 successful to a certain extent, both in saving fuel 

 and in preventing smoke ; but their expense, com- 

 plication, and liability to get out of order, have 

 hitherto prevented their coming into extended 

 use. 



The steam-pressures used in boilers vary very 

 much. In common factory boilers, the working- 

 pressure is from 30 to 50 Ibs. per square inch ; in 

 non-compound marine engines, 25 to 35 Ibs. ; in 

 compound engines, 60 to 80 Ibs. ; and in loco- 

 motives and traction-engines, 100 to 150 Ibs.* In 

 order to prevent waste of heat by radiation, all 

 boilers should be covered with some non-conduct- 

 ing substance. Bricks are often used for this 

 purpose, and felt ; and there are various composi- 

 tions which can be laid on like mortar, and harden 

 in their place. The saving in fuel from covering 

 boilers exposed to the air is very great, often as 

 much as 20 per cent. 



In some cases the steam, after leaving the boiler, 

 is heated to a temperature higher than that corre- 

 sponding to its pressure ; it is then called ' super- 

 heated' steam, and the heating apparatus a 'super- 

 heater.' Superheating increases the efficiency of 

 the steam, and, when used moderately, has no bad 

 results ; but the steam, when too ' dry,' is found to 

 injure the working surfaces in the valve-chests 

 and cylinders. In almost all marine engines, the 

 steam is slightly superheated. 



Having now endeavoured to explain the theory 

 of the action of the steam-engine, and the nature 

 of the apparatus employed for the production of 

 steam, we pass on to consider the engine itself ; 

 and before entering on practical details, we think 

 it well to give a few historical notes shewing 

 the rise and progress of the invention. 



It appears that the first historical mention of 

 the action of steam for producing motion (though 

 not then proposed to be applied to practical pur- 

 poses) was in the Pneumatics of Hero of Alex- 

 andria, in 130 B.C. His instrument was denomin- 

 ated an (zolipyle (fig. 6), and it may be considered 

 the original form of the steam-engine. The aeoli- 

 pyle consists of a globular metallic vessel, A, 

 which rests on pivots, on which it can revolve 

 with perfect facility. Two tubes, E, E, closed at 

 their extremities, proceed from the ball at right 

 angles to the pivots, and each has a small aper- 

 ture at F, near its end, through which the steam 

 can escape. The pivots are the extremities of 



* These pressures are all expressed in pounds above the atmos- 

 pheric pressure. 



