Another important contribution of M. Cailletet is an apparatus for measuring the height of the balloon by photography in order to verify Laplace's formula connecting the barometric pressure with the altitude. The idea was to replace the observers on the ground, who sometimes made the trigonometrical measurements of the balloon described, by a photographic apparatus carried by the balloon itself, and which at frequent intervals should photograph automatically the ground over which it passed, at the same time that an aneroid barometer was photographed on the same sheet. The apparatus is hung vertically below the balloon; in the lower portion of the box is an objective which photographs the ground, and in the upper portion is a second objective which photographs the face of an aneroid barometer placed at the proper distance above. A clock-movement makes exposures every two minutes, and a sensitive film unrolled between the objectives receives the images on each side. If there are known, the focal length of the objective, the distance of two points on the ground, and the distance of two points on the photograph, a simple proportion permits the height of the balloon to be determined at that time, and consequently, from the barometric record, the law connecting the pressure with height can be deduced. The apparatus was successfully used in the voyage of a large balloon with observers, and the accuracy of the determination of height was found to be within 1/250. If the apparatus is to be used at great heights it would be necessary to protect the barometer and the camera from the very low temperatures. Besides the use for which it was designed, this apparatus may serve to trace the route of a balloon and to determine the horizontal velocity at the different points of its path.