Chap. XX.

WARMING AND LIGHTING.

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compressed, and then passes into a more or less compounded state of motion. Birds usingtheir wings as oars in the water find in the air a fulcrum to raise themselves by; theyowe this faculty of flying to a muscular force which, compared to their mass, is superiorto that of all other animals.

Parachutes in unfolding embrace a large volume of air, compress it, and communicatemotion to it : the initial velocity due to weight decreases rapidly, and a mass of from 80to 100 kilogrammes fixed to a parachute is soon reduced to that of a light body, such as afeather, which floats bv itself in calm air.

The vanes on the fly of a turnspit experience, when turning in the air, a resistancewhich equalises the accelerating force of the moving weight, and by this means we regulatethe motion of the endless chain which supports the spit; this regulator is also used inblock-makino\ The effects of the air’s resistance have been observed with care by phi-losophers studying gunnery : we shall show the results of their experiments when treatingof gases employed as prime movers.

CHAP. XX.

WARMING AND LIGHTING.

Warming an Apartment .—We have already seen that, among the Romans, air was heatedby passing over a small fire, and being then introduced into square earthenware tubes, whichwere made to circulate around.the walls of their houses; these tubes, after beating the ■walls,by their radiating effect, raised the temperature of the rooms, or allowed the air so rarefiedto pass from apertures in the pipes into the apartments, thus diffusing a warmth throughout:many of their villas discovered in England show the subterraneous furnaces, and we haveno instance of the open fireplace much earlier than in the keep of Coningburgh Castle,built soon after the Conquest, the mantels of which are formed of flat arches, very inge-niously built, and bearing some resemblance to others constructed at an earlier period inthe south of France .

Radiation, or the properties of heat, does not seem to have been understood, as applied tofurnaces, till the end of the last century, when Count Rumford drew attention to thesubject: the open fire was preferred in Britain at a very early period, and has continuedto be so up to the present day, notwithstanding the waste of fuel and expense attending it.In the first cabins, and the halls of the Norman barons, the fire was made on a hearth in thecentre of the hut or place to be warmed, the smoke generally finding its way through ahole in the roof, examples of which were remaining till lately at Penshursf Castle andother ancient houses.

The blazing hearth, like our open fires, contributed its heat by radiation, and it is wellknown that bodies will throw off heat when their temperature is far below that requiredfor ignition. Scheele, in a work published about 1777, observed that radiant heat passedthrough air without communicating warmth to it, and that currents of air do not interceptit: before heat can be radiated, it must be absorbed, and rough, unmetallie substances,such as brick, are admirably adapted for this purpose: the heat which passes from anopen fire radiates from the burning coal, as well as from the surfaces surrounding it; thistraverses the air, and warms the bodies upon which it impinges; thus the contents of anapartment, all the furniture, which can absorb heat, so receive it, and again radiate it uponother bodies colder than themselves.

Various substances possess different powers of radiation, and those which are goodradiators when hot absorb it quickly when cold: from Mr. Leslie’s experiments, takinglampblack as equal to 100, crown glass is 90, plumbago 75, clean lead 19, and thepolished metals from 12 to 15, proving that polished metals are bad radiators, and not suchgood recipients of heat as those surfaces which are not metallic. Sir Humphry Davy says «that vessels intended to retain their heat should be metallic and highly polishedupon scientific principles, independently of elegance and delicacy. Steam or air-pipes forwarming houses should be polished in those parts where the heat is not required to becommunicated, and covered with some radiating substance, such as lampblack, wherever itis to be diffused. Culinary implements should be blackened, and not polished, on thoseparts which are to receive the heat: the surfaces of fireplaces or stoves should not bemetallic, but of stony or earthy materials, which will communicate much more heat byradiation. The sun and an open fire both communicate heat upon this principle, and theworst conductors, as lampblack, ground glass, &c. are often the best radiators.

Heat which radiates in right lines is also susceptible of being reflected: if a cannonball made red-hot be placed in the focus of one mirror, and an air thermometer in another,

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