DRUMMOND'S LIGHT. The difficulty of distinguishing the stations chosen for the angular points of the triangles in a geodetical survey, such as the Ordnance Survey of the United Kingdom, when those stations are many miles asunder, renders it necessary to have recourse to illumination even in the day-time ; and the late Captain Drummond, of the Royal Engineers, invented a heliostat which reflected the sun's rays in sufficient abundance to render visible the station which was to be observed.
This was a plane mirror of a rectangular form and mounted on a stand with joints by which it could be fixed at any angle with the horizon. On the stand was a telescope which was capable of being moved horizontally, with the mirror, and directed to the distant station while another telescope was directed to the sun. The adjustments of the mirror were such that, when the telescopes were directed as has been said, the face of the mirror reflected the rays of the sun on the distant station, and illumined it sufficiently to render a mark there visible in the telescope of the theodolite by which the required angle was to be taken.
In order to observe the angles subtended between distant stations at night, Captain Drummond employed a light which has been found to exceed in brilliancy any before used. This is produced by placing a ball or disk of lime, about a quarter of an inch in diameter, in the focus of a parabolic mirror, at the station to be rendered visible, and directing upon it, through a flame arising from alcohol, a stream of oxygen gas.
The cistern containing the alcohol is sup ported on a stand, behind the reflector, and is connected by a tube of caoutchouc with the lower part of a hollow stem supporting the upright wire at the top of which is fixed the ball of lime on a level nearly with the cistern: the spirit ascends in the stem, and afterwards through three or more tubes to the ball.
The vessel containing the oxygen gas is con nected, by a flexible tube, with an orifice in a cylindrical box on the same stem, from which it ascends through three flexible caoutchouc tubes to the ball, after passing with friction through three small cylinders. • The whole apparatus is attached to a stand which carries the mirror; and adjustments are provided by which the ball may be placed exactly in the focus of the mirror. The intensity of the flame is from sixty to ninety times as great as that of an argand burner, while the expense is only about ten times as great. The lime made from chalk is preferred to any other ; and such is the brilliancy, that stations above sixty miles from one another have been very distinctly visible even in hazy weather.
Captain Drummond suggested that burning lime should be employed for lighthouses ; and he proposed that, instead of alcohol, hydrogen gas should be employed with the oxygen gas. The gases are to proceed from separate ves sels, or gasometers, and enter a chamber through a series of small apertures : the united gases are then to pass through two or three pieces of wire gauze, and issue in two streams against the ball or disk of lime. To prevent the latter from wasting too rapidly in one place, it is made to revolve once in a minute; and in order to keep up a constant light, it is proposed to have an apparatus by which a number of balls may be successively made to fall in the focus of the mirror.
A light of this kind may also be employed as a signal in determining the difference between the longitudes of stations.
The Electric light, brought under public notice since Captain Drummond's death, has not yet, we believe, been applied to geodetical purposes.