Chap. 2.]

483'

Experiments with a Sheet of Paper .

fairies, those mischievous imps who, in their wayward moods, sometimesundertook to blow the fires as well as to sweep and sand the floors of thehouses they visited, and who, by their screams of delight on leaving theirwork, were supposed to produce the hollow sounds in the flue as theydarted up to join their comrades in the terapest without! It need hardlybe observed that it is gusts of wind, sweeping in particular directions overthe tops of chimneys, and thereby eausing a partial vacuum within them,that thus powerfully increases the draft. But it is not necessary to havefire in the grate, for the effect may be noticed in parlors during the summermonths, when those light and ornamental paper aprons with which ladiescover the fronts of their grates are often thus drawn into the flues, and be-come disfigured and spoiled.

Other examples may be derived from the movements of interior doors,blinds and curtains of Windows , &c. While we are writing, the frontdoor of our dwelling is opened, which affords a clear passage from theStreet to a garden in the rear. The door of the room we occupy opensinto the passage, through which a flaw of wind has just passed, and in atwinkling the blinds swing from the Windows , and the door is slammed toits frame, by the air in the room rushing to join the passing current, or tofill the slight vaCuum produced by it. An open fire-place creates a draftup the ehimney, which acts as a pump to draw cold air into the room;hence the complaint, not at all uncommon, of being roasted in front whilefacing the fire, and at the same time experiencing the unmitigated rigorsof winter behind. (In such cases the combustion should be supported byair drawn from without by a pipe terminating beneath the grate—a devicepatented in modern days, though it was known two centuries ago, and isdescribed by M. Gauger in his treatise on “ Fires Improved,” a worktranslated by Desaguliers in 1715.’-) The motion of every object in natureproduces currents of air, and in every possible direction—the movementof the hand in writing or sewing—the trembling of a leaf or of an earth-quake—the flight of an eagle or of an insect—the ball whizzing from acannon’s mouth, the creeping of a snail, or a wasp using her forceps.

Artificial illüstrations might be quoted without end. Lay two books ofthe same size, or two pieces of board, six or eight inches apart upon afable, and place a sheet of paper over them; then blow between the books,and the paper, instead of being displaced by the blast, will be presseddown to the table by the atmosphere above it, and with a force propor-tioned to the intensity of the blast. Instead of the fnouth next use a pairof bellows, by inserting the nozzle under One edge of the paper, and theeffect will still be the same. The stream of wind may even be directedpartly against the under side of the paper, which notwithstanding will re-tain its place and be pressed down as before. Süspend the books or fixthem to the Under side of a table, then hold on the paper tili the blast isapplied, when the sheet will be sustained against gravity. Fold the paperinto a tube and blow through it with the mouth, or with bellows—in bothcases it will be collapsed. From this experiment we learn that the forcewhich fluids exert against the sides of pipes that contäin them, is greatlydiminished when they pass rapidly through. We have known a smallleak in the pipe that supplied steam to a high-pressüre engine, cease togive out vapor every time the communication was opened to the cylinder

a “ Parabolical jambs ” (also patented) or baeks of grates for reflecting from their po-lished surfaces the heat into the room, are described in the same interesting iittle work.At page 140 Desaguliers speaks of bellows invented and patented by Captain Savery—a device of his that- is rto where eise meiitioned that we are aware of.