a = q/v = 2,000∕37,980 square feet. = 2,000 × 144∕37,980 = 7·58 square inches. This gives the required size of outlet. The size of inlet and outlet together = _15·16 square inches._ _If 3,000 cubic feet of air are supplied in one hour through an aperture of 12 square inches to a room containing 1,000 cubic feet of space, at what rate does the air enter the room?_ 12 square inches = 1∕12 square foot. a = q/v 1∕12 = 3,000/v Therefore v = 36,000 feet per hour. = _10 feet per second._ _If a room is supplied with 3,000 cubic feet of air per hour, through a single opening, what must be its area, if the rate of movement of the air is 5 feet per second?_ 5 feet per second = 18,000 feet per hour. a = 3000∕18000 = 1∕6 square foot. = _24 square inches._ As already stated, the difficulties connected with the estimation of amount of friction greatly detract from the practical value of the formulæ just given. Even the results given by anemometers are not always trustworthy, but by comparing the results given by them with those obtained by the use of Montgolfier’s formula an approximation to the truth can be obtained. The ordinary _anemometer_ consists of four tiny vanes fixed to a spindle, so that revolutions are caused by the current of air the velocity of which is to be measured. The revolutions are counted by a mechanical arrangement. The value of the revolutions of the vanes has to be first determined by direct experiment; a known bulk of air being forced through a channel of known size at a uniform rate, and the instrument graduated accordingly. In Fletcher’s anemometer a modification of the manometer or pressure-gauge has been used for the same purpose. =Inlets and Outlets.=—Having given the average velocity of the wind, the size of a room, and the number of persons occupying it, the size of inlet opening required can easily be calculated. _Find the size of inlet for air in a room occupied by one person, the air moving at the average velocity of 5 feet per second, assuming that 3,000 cubic feet of air are to be supplied per hour._ Let x = size of inlet. Then x × 60 × 60 × 5 = 3,000. Therefore x = 3000∕18000 = 1∕6 square foot. = _24 square inches._ _Given that the air moves at a velocity of 10 feet per second, and that the area of the inlet aperture into a room is 12 square inches, find how much air enters the room in an hour._ Let y = amount of air. Then 10 × 60 × 60 × 12∕144 = y. Therefore y = 3,000 cubic feet of air. Calculations as to supply of air in a room founded on the _average_ velocity of air-currents are, however, much less trustworthy than when the velocity is determined, as previously explained, by means of Montgolfier’s formula, or, better still, by an anemometer.