AERIAL PHOTOGRAPHY.—In the year /858 Nadar, of Paris, made photographs from the car of a balloon. A balloon camera for obtaining photo graphs from a great height was invented and patented by Woodbury in 1877. To a captive bal loon was suspended a camera, fitted with a rapid lens and an instantaneous shutter. Inside the camera were two rollers carrying a hand of sensi tive tissue moved by clockwork. This clockwork and the instantaneous shutter were both controlled by an electro-magnet communicating with the operator on terra firma by electric wires running inside the cord to which the balloon was attached. By means of a small battery the operator below sends a current of electricity through the wire to the electro magnets. The actions of the latter are first, to release the clockwork and bring into position a piece of sensitive tissue or film, and, '', secondly, to release the shutter and make the exposure. This operation is repeated until the whole of the sensitized tissue contained in the camera is exhausted, when the balloon is drawn to the earth and the images upon the exposed tissue developed. The uses of an instrument of this kind are many. For war purposes it would be invaluable, enabling one to photograph and ascertain with certainty the position, strength, and movements of the enemy. For exploring expeditions, the advantages to be reaped by the use of such an instrument cannot be over estimated.
For similar purposes, M. Batut, a clever French amateur, has invented a kite camera, the following description of which was given in the Photographic Times : " The kite is diamond shaped, with a long tail, assuring stability when it has ascended into the regions of the clouds. The cord holding it to the earth is attached to the frame by a sort of trapeze, so arranged that the camera A (see Fig. 20), which is also fastened to the frame by the triangular support D, shall have an unobstructed range of whatever is below. The shutter, which is of the " drop " variety, working horizontally, being actuated by two rub ber bands, is liberated by the burning of a fuse, C, which burns through a thread holding the shutter on tension ; the thread being burned releases the shutter, which, in its flight across the opening of the lens, also liberates a scrap of paper which, floating down, shows the manipu lator that the exposure has taken place. A self-registering barometer, B, is attached, to show the altitude attained. The cut (Fig. 21) shows a result obtained by M. Batut, at an altitude of 127 metres, on February 13, 1889, at i i A.M. It represents a plan view of a farm-house, with its outbuildings. The kite employed is about 7 feet in height ; the camera weighed 1,200 grams.
Another curious form of aerial photographic apparatus was developed by a French inventor, M. Denesse. It consists of a photographic camera attached to a rocket. An umbrella
like parachute is also fixed to the rocket-stick. When fired into the air this is closed, but as soon as the rocket begins to descend it opens out, and the whole returns gracefully to the earth.
In this the camera is cylindri cal in form, and has round its circumference twelve lenses— a sensitive plate is in the centre.
The lenses are provided with a shutter which opens and closes instantly on the camera com mencing to descend. It is then drawn back to the operator by a cord attached before the fir ing of the rocket. The prin cipal advantages of this form of apparatus are cheapness of operating and freedom from risk.
Many other methods of making photographs from a height have been devised and some very useful results ob tained. It is not only for war and exploring purposes that here pictures become valuable: students of geodesy will also find them of service in the construction of plans and maps. Dr. Frank Stolze says that it may be confidently assumed that during the exposure the sensitive plate on which the photograph is made has hardly ever been horizontal enough to allow the plate to be considered analogous to a parallel projection of the natural landscape ; it is much more likely to be always more or less perspectively foreshortened There exists, however, a particularly simple and reliable means of finding proper geometrical projections from the perspective projection. This is done after the following manner : An exact square of large dimensions—say 600 feet—is described upon a flat part of the ground to be photographed, and the four corners marked distinctly, so that they shall be plainly seen in the photograph. From the now perspectively foreshortened image of the square we can find with ease all the constituents of perspective distortion, so that it is possible by working out the perspective backward to find for every point of the picture the corresponding point of the geometric construction, which is then, when we have to deal with level ground, a comparatively easy matter. But when, however, the ground is hilly and undulating, the matter becomes a more difficult and complicated one. It is then necessary to take as auxiliaries some ordinary photogrammetric views in order to find out the difference of levels, and to reduce all the data on the photograph to the same horizontal. Notwithstanding this, however, the work will be far easier than when one has to trust to terrestrial photogram metry ; and it may, indeed, be stated that views taken from a height and used in this manner far exceed in simplicity and ease all that has yet been done in the way of special views taken for purposes of geodesy. In fact, views taken from balloons, parachutes, kites, or rockets, are undoubtedly the geodesic views of the future.