Page:Popular Science Monthly Volume 53.djvu/75

Rh changing their tint with every motion, the casual observer has an impression that he is gazing at a flock of real birds, with a notion of white, gray, pink, blue, and purple gulls, uncommonly wide of wing and unusually fascinating.

Now and then one or another sails away from its place and poises itself for a few moments, then returns nearly to its former position; while another takes a sudden dive downward, ending in a graceful parabola which brings it to a new point on its original level. Sometimes one kite will move almost directly at another, which shyly sidles away; when the first ceases its movement, droops, and sinks down, until, just in the nick of time, a strong young zephyr catches it and buoys up its faltering pinions.

The cause of all these apparently purposed movements being invisible to the beholder, some degree of reflection is required to rid one's self of a lurking idea that these are animate things.

The lifting surface of these several kites is assumed to be the total surface of the side planes (upper and lower). The sticks have a rectangular or elliptical cross-section.

The kites are very stable, and fly in recorded wind velocities of from six to twenty metres per second. The angular attitudes reached by the first two kites average between forty-five and fifty-five degrees, and those reached by the last two average between fifty and sixty degrees. The pull in a recorded wand of ten metres per second averages about five kilogrammes per square metre of lifting surface.

The Blue Hill method of testing kites is to fly them with a short line, usually from fifty to one hundred metres long, and to make frequentand regular observations of the angular attitude and of the pull. The instruments employed are a surveyor's transit and a spring balance. Tests are usually made under widely varying conditions of wind velocity, and the kites flying at the highest angles and those exerting the greatest pull are easily selected. The work in view consists of raising a meteorograph of known weight under varying conditions.