274 



THE POPULAR EDUCATOR. 



heat ia thus imparted to the water, that there is little need for 

 external flues. The water is also kept in this way in more con- 

 stant circulation, which is a very important thing. In this 

 form of boiler there is a much larger amount of " flue surface," 

 as that portion which is directly exposed to the hot air is called, 

 and more of the heat is consequently utilised. There are many 

 other forma of tubular boilers, but we cannot specify them 

 here. 



In the construction of any boiler it is very important to take 

 care that the flue surface is always below the level of the water, 

 as then it is kept from attaining too high a temperature. If the 

 fire or hot air acted on any part of its surface which was above 

 the water level, the plates would become unduly heated, and 

 could not withstand the pressure, but would bulge or give way. 



A very large amount of heat is often lost by radiation from 

 the surface of a boiler ; to guard against this waste, it is some- 

 times covered with felt, or surrounded by a jacket filled with 

 sawdust, which is a good non-conductor. Locomotive boilers 

 and the cylinders of engines are also frequently covered with 

 wood for the same purpose. 



In order to maintain a sufficiently high temperature in the 

 furnace, it is necessary that there should be a powerful draught. 

 Some means of regulating it is also requisite. The latter may 

 easily be provided by means of suitable dampers placed in the 

 flues, so as partly to stop the current of air. For fixed boilers 

 a tall chimney is often erected, and a sufficiently powerful 

 draught may usually be secured in this way. Artificial expedients 

 are, however, frequently resorted to, especially in the case of a 

 locomotive or marine boiler, where the chimney is of necessity 

 short. The most common of these expedients is to place a small 

 steam-jet in the. chimney, and the force of the steam escaping 

 from this creates a powerful draught. The waste steam, after 

 leaving the cylinder, is usually thus employed, and this produces 

 the succession of puffs which are so often observed issuing from 

 the funnel of a railway-engine. 



If the fire be made too fierce, or the machinery stopped, so 

 that no steam is drawn off from the boiler, it soon accumulates, 

 and its pressure would in a little time become so great as to 

 burst the boiler with terrific violence, were not some means pro- 

 vided for allowing it to escape. This is accomplished by means 

 of a safety-valve, the action of which will easily bo understood 

 by reference to Fig. 4. A A represents a portion of the upper 

 surface of the boiler ; in this is inserted a short tube of small 

 dimensions, the upper end of which is bevelled off so that a 

 plate, B, may fit it accurately, and close it, so as to prevent the 

 escape of the steam. A spindle attached to B causes it to rise 

 and fall vertically, and it is pressed down by the weight, w, 

 acting on the end of the lever, c D. The pressure on it can be 

 adjusted by altering the position of this weight. Sometimes, in- 

 stead of a weight, a spiral spring is placed at the end D, and the 

 pressure is then altered by means of a screw fitted to it. In 

 most boilers two safety-valves are used, one of which is locked 

 so as to be quite out of the control of the engineer. This is 

 adjusted so as just to open with the highest pressure which the 

 boiler will safely bear. The other is adjusted by the engineer, 

 so as just to sustain the pressure at which he is working. 



As the boiler is used, a portion of the water in it becomes con- 

 verted into steam, and consequently the level falls. Were this 

 allowed to continue, it would soon become so low that the flue 

 surface would not be protected by being covered with water, 

 and ^the result would be that the plates would be softened by 

 the intense heat, and thus be unable to withstand the p'ressuro 

 of the steam. Some contrivance is therefore required to main- 

 tain the water at a uniform level, and also to indicate at once if 

 by any accident the water sinks below this level. The pressure 

 on the surface of the water in the boiler is, however, very con- 

 siderable, and the fresh water to be supplied must, therefore, 

 be forced in with a greater pressure. 



Now, as we shall see shortly, there are two classes of engines, 

 known respectively as low-pressure and high-pressure engines. 

 In the former the pressure in the boiler is usually not more 

 than from eight to twelve pounds to the square inch ; in the 

 other it is very much greater. 



A low-pressure boiler is easily kept supplied with water by 

 placing a cistern at such a height above it that the column of 

 water from it to the boiler is longer than can be sustained by 

 the presauro of the steam. From this cistern a pipe, closed by a 

 valve at the top, leads to the bottom of the boiler. A float in 



the boiler is then attached to one end of a lever, the other end 

 of which moves this valve, so that as soon as the float falls the 

 valve is opened, and a fresh supply of water ia admitted to the 

 boiler till the level is restored. In high-pressure engines a small 

 force-pump is added, by means of which water can be injected 

 into the boiler when required. This can easily be thrown out 

 of gear when not wanted. 



Two cocks are usually placed in one end of the boiler, one at 

 the highest, and the other at the lowest level which the water 

 may safely reach. When there is any doubt about the quantity 

 of water in the boiler, the engineer opens these. Steam ought 

 to issue from the upper one, and water from the lower one. If 

 steam comes from both, it is a sign that the water has been 

 allowed to sink too low, and a fresh supply should immediately 

 be admitted. 



In addition to these, a glass water-gauge ( Fig. 5) is usually 

 placed in the boiler. This consists of a tube of stout glass, A, 

 fitted into brass caps, B, B, which communicate with the interior 

 of the boiler. Cocks, c, C, are provided, by means of which con- 

 nection with the boiler can be cut off at pleasure. When these 

 are opened, the level of the water in the tube will, of course, be 

 the same as that in the boiler, and hence it can be seen at a 

 glance. The water can be emptied out of the gauge by opening 

 the cock D, placed below the glass tubel 



Usually the boiler is fed with the condensed steam which is 

 allowed to accumulate in the hot well. The temperature of this 

 is high, and a considerable saving in fuel is effected by using- 

 it instead of cold water. A very important thing in the 

 management of the steam-engine is to secure the greatest 

 amount of power with the least expenditure of fuel, and many 

 alterations have been suggested with a view of attaining this 

 end. The dark smoko which may often be seen issuing from 

 the chimney is in reality unconsumed fuel, and therefore, 

 besides being a nul. ance, is a positive loss. Many different 

 kinds of furnaces havo been tried, with a view of completely 

 consuming the smoke ; a very great deal, however, depends 

 upon the mode in which the fresh fuel is supplied. When it is 

 shovelled in in large quantities, smoke is sure to be produced. 

 The proper plan is to press back the fire a little way from the 

 furnace-door, and lay the fresh fuel in front. The smoke pro- 

 duced has then to pass along the surface of the intensely-heated 

 fire, and is thus consumed. By carefully feeding in this way, 

 and introducing the fuel in small quantities and at frequent 

 intervals, almost all smoke is avoided. In self-acting arrange- 

 ments the coal is usually thus introduced, and the furnace-bars 

 travel slowly backwards, carrying the fire with them. 



There is only one other adjunct of the boiler to which w 

 need now refer. It is an important thing for the engineer to 

 bo able to tell the pressure of the steam on the boiler at any 

 time, and for this purpose a small pressure-gauge is attached to 

 a convenient part of the boiler. A hand on the dial-plate of 

 this indicates the pressure in pounds on the square inch of 

 surface. 



Volvi a verle, I saw him again 

 (literally, I returned to see him). 



LESSONS IN SPANISH. XXII. 



IDIOMATIC USE OP CEKTAIN VERBS. 



Volver a, to return, to repeat, is used before an infinitive 

 when it is required to repeat the action denoted by the infini- 

 tive, in which case the adverb again would be used in English, 

 and the infinitive be rendered in the same tense as volver ; as 



Volvio a escribir la carta,* 7i 

 re-wrole (or wrote again) the letter. 



Acaber de, to finish from, is used before an infinitive in the 

 sense of to have just, and the infinitive is rendered in English as 

 a past participle ; as 



Juan acaba de llegar, John has I Acabo de verle, I have just seen 

 just arrived. j Tun. 



Estar para, to be towards, is used before an infinitive in the 

 sense of to be ready, or to be about to; as 



Estaban para acabar sus estu- They were about to finish their 

 dios, ttudies. 



* Literally, Tie returned (or repeated) to write the letter. It must be 

 kept in mind that this is the usual mode in Spanish of expressing the 

 repetition of an action, instead of using a word corresponding to ajain. 

 in English. 



