Chap. XX.

WARMING, LIGHTING, AND VENTILATION.

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heat properly applied has been found most efficient. Air at an ordinary temperature ex-pands part for the rise of every degree of the thermometer : thus it becomes, as itadvances in temperature, specifically lighter, and consequently has an inclination given to itto mount higher, and give place to the colder air, which sinks on account of its greaterdensity: this takes place in the common chimney, the draught of which depends on thedifference of temperature at the top and at the bottom; the air at the lower end, by beingheated, rises and passes off at the upper, its place being supplied by the pressure of thesurrounding air to fill up the vacuum. Supposing v to denote the velocity in feet per se-cond, (j the accelerating force of gravity, (j=S 22 feet per second,) a the rate of expansionfor 1° of the increased temperature, t' the temperature of the heated air as it enters thechimney, and t, that of the external air, we shall have this formula,

» = •/ {‘2gha(t '—*)],

the velocity of the draught being as the square root of the height of the chimney, and thesquare root of the excess of temperature of the lower opening : and as the same quantity ofair passes through every section of the chimney in the same time, the velocity must beinversely as the density, and therefore decrease from the bottom to the top.

Air forced into a chamber by mechanical means is often used for the purposes of ventila-tion : at the Hank is a square box, furnished with a pump with a loose piston, which haslarge-valved openings, and by it 11,000 cubic feet of air per minute are driven through thevarious offices; at the Houses of Parliament a large shaft, with a fire made in it nearits summit, exhausts the air from the several apartments, in the same manner as incoal mines, and other deep workings.

The practice adopted in deep mines to obtain ventilation seems to have given rise to allthe modes now made use of in our buildings for the same purpose : suppose there aretwo shafts, which descend to the bottom of the mine, one is called the downcast, the otherthe upcast shaft; by the first the air descends, and by the other the vitiated air mountsand is dispersed. This movement is effected by constantly keeping up a large fire at alittle distance from the bottom of the upcast shaft, and making use of it as a chimney : bythis simple contrivance, the descending air may be made to pass through and along everyworking of thy mine) in some instances, it travels at the rate of 30 miles per hour, drivingall the lighter gases before it; numerous doors are introduced to shut off the course aswell as to direct it; should any of these be left open, the whole system of ventilation willbe destroyed.

Theatres are frequently ventilated by means of a funnel placed in the ceiling of thepit, and having its exit through the roof, upon the top of which is a movable cowl: byplacing the large chandelier, or a multi-tude of gas-lights, immediately belowthe opening in the ceiling, the air aboutit becomes so rarefied, that it rapidlymounts the funnel, and passes outat the mouth: generally this part ofthe operation, viz. the exhausting orpumping out, is tolerably well effected,but not so the admission of a warm andpure air to supply the wants of thespectators, each of whom requires airfor 20 respirations per minute.

Lighthouses may be classed amongthe works of the greatest public utilityand importance to a maritime nation :we have already described many erectedafter the Eddystone; the form at presentusually adopted, when on land, is thatof Ramsgate Harbour, which is circularon the plan throughout, the rooms forthe attendants being on the ground floor,and a winding staircase conducting to thelantern.

A Select Committee appointed by theHouse of Commons made a report in1834 upon the state and managementof lighthouses, floating lights, buoys,and beacons, from which we gather themost valuable information that could beafforded: to Joseph Hume , Esq., the

chairman of the committee, the com- Fig. 1928. ramsgate lighthouse.