Such a method exists for registering the movements of this class of instruments, the details of apparatus and manipulation being altered to suit each individual case. There are, however, other instruments to which such would be totally inapplicable. As an example, we may take the magnetometer. The oscillations of the suspended magnet as used for measuring the horizontal or vertical components of the earth’s magnetism are very minute, so minute indeed that they can scarcely be perceived by the eye. If to one of these magnetometers, however, we attach a very small and light mirror, the plane of which is at right angles to the axis of the magnet, and cause a beam, proceeding from a source of light, to pass through a small aperture, thence to a fixed lens on to the mirror, which reflects the beam of light on to a screen so placed that the image of the aperture is in the focus of the lens, any small deviation of the magnetometer will cause the beam of light to deflect on the screen. The amount of the deflection will be dependent on the focal length of the lens, and the distance of the aperture and screen from the mirror. Suppose the screen to be opaque, and that a slit is cut in it in the direction that the deviation of the beam would take, and lying in the same plane as the deviation, and that a strip of sensitive paper moves behind that slit in a direction at right angles to its length, then at each instant the position of the beam of light will be registered on the paper. On developing the image we shall have a sinuous line corresponding to the deflections of the magnetometer at every time of day and night, the reading of the time being dependent on the rate at which the paper travels. For meteorological purposes we may also hope that photography will be more utilized than it has hitherto been. Mr. A. Mallock, at the meeting of the British Association at Plymouth, has shown a way in which it may be made subservient to ascertaining the heights of clouds. In military science it is only necessary to call to mind the service that the pigeon post performed during the siege of Paris. A large series of letters were printed on one sheet, and then photographed to a very small scale on collodion pellicle. Such pellicles, measuring about 6×2 centimeters, were tied to pigeons, which when liberated carried the dispatch to Paris, where they had been trained. On arrival the collodion pellicle was detached from the pigeon, placed in a lantern, and the letters transcribed and sent to the various addresses. Of so much use was this pigeon post that the German military authorities have established a regular service of pigeons in the chief fortresses of the empire, which would be used in case of investment or siege by a hostile army.