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. [Illustration: FIG. 12.--Meteorograph lifted by Kites at Blue Hill.] The method of making a kite-flight for meteorological purposes at Blue Hill is as follows: a kite, fastened by a long wire to the ring in the main wire, being in the air, and the meteorograph suspended, another kite is attached to the ring by a shorter cord (Fig. 12). They are then allowed to rise, and to unreel the wire, until its angle with the horizon becomes low, when, by means of the clamps described, other kites are added, the number depending on the size of the kites and the strength of the wind. After a pause at the highest attainable altitude, the winch is connected with the steam-engine and the kites are drawn down. The pauses at the highest point, and when kites are attached or detached, are necessary to allow the recording instruments to acquire the conditions of the surrounding air; and because at these times the meteorograph is nearly stationary, measurements of its angular elevation are made with a surveyor's transit, while observations of azimuth give the direction of the wind at the different heights. The time of making each angular measurement is noted, so that the corresponding point on the trace of the meteorograph may be found. From the length of the wire and its vertical angle, the height of the meteorograph can be calculated, it having been found that the sag of the wire, or its deviation either in a vertical or a horizontal plane from the straight line joining the kite and the reel, does not cause an error exceeding three per cent. in the height so computed. When the meteorograph is hidden by clouds, the height above the last point trigonometrically determined is computed from the barometer record by Laplace's formula. At night there is only the barometer from which to determine the height; for although an attempt was made to use a lantern to sight upon, yet it