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THEORY AND PRACTICE OF ENGINEERING.

Book II.

these have seven parallel cast-iron cases, 3 inches in depth; into these cases the steamis conducted by proper pipes, and between them the air to be warmed passes. Thesteam-engine, of 5-hor*e power, which is on the expansive principle, serves other purposesthan that of turning the ventilator, and consumes about 2 cwt. of fuel in twelve hours.

This principle is the reverse of that already described as adopted at the New Model Pri-son : here we have genial air thrown in by the fan, and its circulation insured by pressurefrom below, and not by exhaustion from above ; the pure cold air is obtained, and raised toan agreeable state of temperature, after which it is dffused into all the halls, passages, androoms around them. When air is pumped out, and no abundant supplv forced to succeed,what remains in an apartment is likely to be so highly attenuated as to prove injurious tohealth: when attention is paid to the ventilation and passing off the vitiated air, the pressuresystem is preferable. Fans of sufficient size may be driven by means of steam power withsuch velocity as to produce ventilation throughout the most extensive buildings : condensedair has a much more agreeable and healthy effect upon individuals than can be obtainedfrom highly rarefied air : a person breathing condensed air does so with facility ; he feelsthe capacity of his lungs enlarged, and his respirations are less frequent; in rarefied airthe breathing is attended with difficulty, and in consequence of this, in some of the deepmines in France , the men work in condensed air pumped down from a chamber above :when these workmen are about to descend they shut the door over their heads, and thenturn a cock, which is in connection with the condensed air on the under shaft: an equili-brium is established in an ante-room, by the entry of the dense air from below, and thenthe man-hole can be opened for the men to descend : here they work in air maintained ata pressure of three atmospheres by the action of pumps driven by an engine ; in suchan atmosphere the miners gain additional strength, and work without fatigue. Thefanner or blower is made in various forms; the most common is that with four arms orblades, working in a box ; the air entering in the centre is discharged at the circumference ;in some of the most powerful, the air is made to enter at the sides of the blades throughouttheir whole extent, and to discharge it at its circumference; by unscrewing one side ofthe cylindrical box, in which a fan works, it will be readily seen which way the air proceedsif a rapid motion be given to it ; the external easing is generally made air-tight, and theair is conducted by a pipe to the fanners, at the opposite side to which it is desired it shouldbe discharged.

The larger the fanner, the greater is the economy with which its supply of air can be had,and they are less subject to noise; those making 2000 revolutions in a minute are exceedinglydisagreeable throughout the whole neighbourhood where they are used; quantity of air,and not velocity, is the object to be attained, and for this purpose fanners of from 10 to 20feet diameter, moving slowly, are to be preferred: powerful blasts of air are not requiredfor ventilation, and therefore rapid movement to the fanner can be dispensed with.

Ventilation probably was first attended to in the mining districts, but nothing on thissubject could be effected upon a proper basis until Priestley defined the composition of theatmosphere; no definite meaning could be given to the word ventilation until the natureof the air itself was known ; to him, Scheele, Dalton, and Cavendish, are we indebted for ourknowledge upon these subjects.

Before the employment of many persons in a small space, particularly in mines, the ne-cessity for ventilation was not rendered so apparent: and the practice first adopted there, tothrow in a succession of currents of wholesome air, has been latterly introduced to disturbthe vitiated atmosphere of our crowded halls and ordinary dwelling-houses. The archi-tect and civil engineer have been roused to contrive and invent means by which life may notonly be supported but endured, when confined in rooms robbed of their fair proportion ofoxygen by the modern luxuries of warming and lighting by gas. A man making twentyrespirations in a minute is said to inhale at each 16 cubic inches of air, or 320 cubic inchesin a minute: of this quantity 32 cubic inches of oxygen are consumed, and 25 inches ofcarbonic acid gas are discharged ; so that, if his life endured for 50 years, it has been esti-mated he would inspire 166 tons of air, consume 18£ tons of oxvgen, and discharge nearly20 tons of carbonic acid from his lungs, or, in other words, nearly 5.^ tons of carbon : allow-ing 10 cubic feet of air per minute to each individual, the consumption for 1000 personsassembled in a theatre or church may in some degree be estimated: as may the necessaryquantity for any given number of persons in rooms of different dimensions.

Of the atmosphere 78 percent, is nitrogen and 22 per cent, oxygen, and the quantity ofcarbon and moisture evolved by a single individual in a minute is stated to be 3-27 grainsof carbon, and 3*2 of water: so that 1000 persons would evolve in an hour 28 poundsof carbon, and nearly as much water.

It is not so easy to determine the quantity of air required for ventilation, as this is somuch affected by temperature : l)r. Arnott informs us that air expelled from the lungsvitiates twelve times its bulk of pure air, so that a man vitiates every* minute nearly 4 cubefeet, and agrees that 10 cubic feet of fresh air per minute is the smallest allowance that shouldbe made for each individual. To produce motion in the air, or, as it is termed, ventilation,