THE POPULAR EDUCATOR. 



Wo must now notice the construction of a few common 

 pneumatic machines, and perhaps the most important are those 

 used for blowing. In furnaces for reducing 1 and melting metals 

 it ia found impossible to cause a sufficient degree of heat to be 

 produced unless a large additional quantity of air be forced into 

 the fire, so as to quicken combustion. In mines, too, and under- 

 ground passages ventilation must be carried on by artificial means, 

 and for these and other purposes blowing machines are employed. 

 The most simple of these is the common household bellows, 



so familiar to all. In the 

 lower of the two boards 

 is a circular aperture, 

 over which a piece of 

 board is hinged, so as to 

 act as a common clack 

 valve. When the boards 

 Fig. iS. are separated, the air 



opens this valve, and 



enters, but as they are again pressed together this valve closes, 

 and the air is then forced to escape by the nozzle. With 

 this kind, however, only an intermittent current can be pro- 

 duced, for while the boards are being separated, air is drawn 

 in at the nozzle as well as at the valve, though in a less 

 degree. This was often found to be a serious disadvantage, 

 and therefore two bellows, working alternately, were used 

 in many furnaces. Double-acting bellows are, however, used 

 now in nearly all forges, and these produce a uniform stream. 

 They consist of two ordinary bellows, placed one above the 

 other. When the under board E (Fig. 18) is lowered, the 

 valve c opens and admits the air ; this is forced, by the rising 

 of the board, through the valve B into the upper bellows. A 

 weight A, placed on the top, drives the air with a constant 

 pressure out of the nozzle D. The board F is fixed, and E 



is usually worked by a 

 lever. Though these 

 bellows are powerful 

 enough for a black- 

 smith's forge, when the 

 metal has only to be 

 softened sufficiently to 

 cause it to weld, they 

 will not answer for a 

 furnace for melting iron ; 

 and a fan, driven by 

 steam, is usually em- 

 ployed in this case (Fig. 



19). The air enters at the axle, and is thrown off by centri- 

 fugal force from the edges, and conducted along large tubes to 

 the furnace. 



Another purpose for which these machines are employed is 

 in the winnowing of corn. In former times, and in some places 

 at the present day, the corn, when threshed, is thrown up in 

 air, and the wind carries away the chaff. The plan now adopted 

 is to allow it to fall through a narrow slit, and cause a rapid 

 current of air, produced by rotating fans, to remove the chaff. 

 One great advantage of this plan is that the strength of the 

 blast may be so regulated as not only to remove the chaff, but 

 to separate also the small and shrivelled grains. 



The pneumatic screw is another simple blowing machine, used 

 for purposes of ventilation. It acts on exactly the same prin- 

 ciple as the Archimedian screw, an axle with a spiral flange 

 being made to rotate in a cylinder. This is placed at one end 

 of the tube or shaft, and produces a powerful current, the 

 direction of which depends upon the direction in which the 

 screw revolves. 



This machine is sometimes employed for the ventilation of 

 mines, and is fixed above one of the shafts. A second shaft 

 allows fresh air to pass down it, and replace that removed by 

 the fan, and thus a constant current of air is kept up through 

 the mine. The main galleries below are so arranged, by 

 means of boarding and doors, that the fresh air must traverse 

 the greater part of the niine before it can find its way to the 

 " upcast " shaft, as it is termed. As the air will always find 

 the most direct road, great care is required in the arrangements 

 for effecting this. 



In most mines in England blowing machines are now dis- 

 pensed with, and in their stead a large furnace is placed at the 

 base of one of the shafts. This greatly rarefies the air above it, 



Fig. 19. 



and thus renders it much lighter than that around. It ascends, 

 therefore, and a fresh supply rushes down the second shaft to 

 take its place, and in this way good ventilation may nearly 

 always be obtained. The plan, too, is more simple than the 

 use of fans, and less liable to get out of order. Sometimes the 

 furnace is placed in a recess, part of the way up the shaft ; 

 sometimes, too, only one shaft is sunk, and divided by bratticing 

 into three or more divisions, one for the pumps and working 

 machinery, the other two for the "upcast" and "downcast." 

 This plan is, however, very dangerous, and many of the fearful 

 accidents we hear of in mines are to be attributed to its 

 adoption. 



Ventilation in our houses and public buildings is carried on 

 in a similar way. It is much to be regretted, however, that the 

 principles on which this should be arranged seem to be so little 

 understood or carried into practice. If we hold a sheet of 

 paper near a large fire, we shall soon see by the powerful draught 

 that there is a strong current of air up the chimney, and cold 

 air rushes in at the cracks of the doors and windows to supply 

 its place. A good fire, therefore, adds greatly to the ventilation 

 of a room. As, however, the heated air rises, it is an important 

 thing to have some outlet for this, and an opening into the 

 chimney near the top of the room will usually be effectual. 

 In public buildings the foul air is usually carried off near the 

 roof, and arrangements ought to be made by which fresh air 

 can enter in a number of small streams at different places, in- 

 stead of flowing in a large body through an open door, and thus 

 creating a violent draught. 



The following examples will give the student good practice in 

 the application of the rules in Lessons V. and VI. : 



EXAMPLES. 



1. If the barometer stands at 29'04, what is the pressure of the air 

 on a surface measuring Sin. x 6^in. ? 



2. In Fig. 15, if the length of the graduated portion of the shorter 

 limb be 10 inches, and the mercury rise in it to a height of 5^ inches, 

 at what height does the mercury stand in the other limb, the baro- 

 meter being at 29in. ? 



3. A volume of gas measures 249 cubic inches when the barometer 

 stands at 28'7. How much will it measure at the standard pressure ? 



4. Some gas at a temperature of 155 measures 1 cubic foot ; how 

 much space will it occupy when cooled to 60 ? 



5. 140 cubic inches of air at 60 is heated till it occupies 215 cubic 

 inches ; what temperature has it attained ? 



6. When the barometer was standing at 28'78, and the thermometer 

 at 71, a quantity of gas was found to measure 158 cubic inches. 

 How much would it occupy at the standard pressure and tempera- 

 ture ? 



LESSONS IN FRENCH. LXXX. 

 105. REMARKS ON THE DEMONSTRATIVE PRONOUN CE. 



(1.) Ce, when used as a demonstrative pronoun, is construed 

 with the verb etre, or with a verb followed by 6tre, or with a 

 relative pronoun : 



C'est un poids bien pesant qu'un 

 grand nom a soutenir. 



MONTESQUIETT. 



Ce qui me plait c'est sa mo- 

 destie. LBVIZAC. 



Ce doit etre lui. 



Ce que vous dites est faux. 

 Je sais C6 dont il se plaint. 



A great name is a very heavyweight 

 to sustain. 



That which pleases me is her mo- 

 desty. 



It must be he. 



What (that which) you say is 

 untrue. 



I know that of which (of what) he 

 complains. 



(2.) Ce is used for he, she, they, preceding any part of the 

 verb to be, when that verb is followed by a noun, or an adjec- 

 tive used substantively and preceded by the, a, or an, or a pos- 

 sessive or demonstrative adjective, or any kind of pronoun. 



(3.) Observe that the verb etre following the pronoun ce, is 

 put in the plural when the noun, possessive or demonstrative 

 pronoun following that verb is plural. The pronoun ce, how- 

 ever, remains unchanged : 



C'est un trompeur. 

 C'est la fernme que je cherche. 

 C'etaient mes amis. 

 Ce seraient paroles exquises, 

 Si C'etait un grand qui parlat. 

 MOLIERE. 



He is a deceitful man. 

 She is the woman whom I seek. 

 They were my friends. 

 They would le exquisite words, if 

 a great man were speaking them. 



