Fig. 10.—Modified Hargrave Kite at Blue Hill.
Perhaps the most important factor in the success of the Blue Hill work was the invention by Mr. Clayton of the regulating bridle which is applied to every kite. An elastic cord is inserted in the lower part of the bridle, to which the flying-line is attached, and when the wind-pressure increases this cord stretches, and causes the kite to diminish its angle of incidence to the wind until the gust subsides. A kite can be set to pull only a fixed amount in the strongest wind, when the kite will fly nearly horizontal. We are therefore able to calculate the greatest pull which can be exerted on the wire by all the kites. With this device the kites have flown through gales of fifty or sixty miles an hour without breaking loose or injuring themselves. Another efficient kite which has been used at Blue Hill is the so-called "aero-curve kite" made by Mr. C. H. Lamson of Portland, Maine. As is seen from [Fig. 11], this kite resembles a soaring bird, and it can be taken apart and folded up for storage or transportation.
In general, the angle of the flying lines of the Blue Hill kites is 50° or 60° above the horizon, and in winds of twenty miles an hour the pull on the line is about one pound for each square foot of lifting surface in the kite. Kites can be raised in a wind that blows more than twelve miles an hour at the ground, and as the average velocity of the wind for the year on Blue Hill is eighteen miles an hour, the days are few when kites will not fly there.
The wire to which the kites are attached is steel music-wire, 32⁄1000 of an inch in diameter, weighing fifteen pounds a mile, and capable of withstanding a pull of three hundred pounds. The wire is spliced in lengths of more than a mile with the greatest care, special pains being taken that no sharp bends or rust-spots occur which would cause it to break. To lift the increasing weight of wire, kites are attached at intervals of a few thousand feet, so that the angle may be maintained as high as is consistent with a safe pull, and this is done by screwing on the wire aluminium clamps, to which the kite-lines are fastened. On account of the greater stability and strength of the new kites, the meteorograph is suspended directly from the top kite. The Richard meteorograph, contained in an aluminium cage of about a foot cube, weighs less than three pounds, and it is only necessary to screen the thermometer from the sun's rays to obtain the true temperature of the air, since the wind insures a circulation of air around the thermometer. Another meteorograph, constructed by Mr. Fergusson, records the velocity of the wind in addition to the three other elements, and it weighs no more than the French instrument.
The reeling apparatus is an example of how the same apparatus may serve diametrically opposite purposes. In sounding the deep sea the wire must be pulled upwards, whereas in sounding the heights of the atmosphere the wire must be pulled in the reverse direction. Therefore the deep-sea sounding apparatus has been altered by Mr. Fergusson to pull obliquely downwards, the wire passing over a swivelling pulley which follows its direction and registers on a dial the exact length unreeled. Next the wire bears against a pulley carried by a strong spiral spring, by which the pull upon it at all times is recorded on a paper-covered drum turned by clockwork. The wire passes now several times around a strain-pulley, and finally is coiled under slight tension upon a large storage-drum. When the kites are to be pulled down, the strain-pulley is connected with a two-horse-power steam-engine, and the wire is drawn in at a speed of from three to six miles an hour; but when the kites are rising the belt is removed, and the pull of the kites unreels the wire.
