The inconveniences attending this lamp have led to several attempts to improve it; and, amongst others, M. DELAVELEYE has proposed to substitute a pump having a metallic piston, in place of the leathern valves, which require constant care, and must be frequently renewed. A lamp was constructed in this manner by M. LEPAUTE, and tried at Corduan; but was afterwards discontinued until some further improvements could be made upon it. It has lately been much improved by M. WAGNER, an ingenious artist whom M. FRESNEL employed to carry some of his improvements into effect. In the dioptric lights on the Scotch coast, a common lamp, with a large wick, is kept constantly ready for lighting; and, in the event of the sudden extinction of the mechanical lamp by the failure of the valves, it is only necessary to unscrew and remove its burner, and put the reserve-lamp in its place. The height of this lamp is so arranged, that its flame is in the focus of the lenses, when the lamp is placed on the ring which supports the burner of the mechanical lamp; and as its flame, though not very brilliant, has a considerable volume, it will answer the purpose of maintaining the light in a tolerably efficient state for a short time, until the light-keepers have time to repair the valves of the mechanical lamp. Only three occasions for the use of this reserve-lamp have yet occurred. ~Height of the flame of the Mechanical Lamp.~ The most advantageous heights for the flames in dioptric lights are as follows:-- Inches. 1st Order, 10 to 11 centimètres = 3·94 to 4·33 2d Order, 8 to 9 ...... = 3·15 to 3·54 3d Order, 7 to 8 ...... = 2·76 to 3·15