A few remarks on this method of obtaining light from the combustion of crude petroleum may be added, as the light has been put forward as a cheaper and better substitute for the electric arc. The Lucigen light is produced by burning creosote oil, tar oil, or other heavy hydro-carbons, by means of compressed air in a special form of lamp, and consists of a cylinder at the side of which a steam donkey compressing pump is mounted, or in a more recent form known as the Wells’ light, no separate air compressor is used, but, instead, the pressure is obtained from the water mains or from a small force pump. The cost is stated to be 3_d._ per hour for 2,500 candle-power, requiring three gallons of oil per hour, but is in reality at the present time double this owing to the price of the oil, which, under the most advantageous circumstances, costs on average 2_d._ per gallon. At the Forth Bridge these lights have been found of use in illuminating open spaces, but have not supplanted the electric arc lights which are universally employed for the lighting of the works and the interior of the shops. The disadvantages are the noise, the oil shower which pervades the vicinity of the light causing timber staging to be highly inflammable, and the difficulty of preventing water from entering the burner, a few drops sufficing to extinguish the light. The use of the Lucigen light is, therefore, very limited, and it is probable that, in situations where shadows from the arc light are found to be objectionable, large incandescent electric lamps, which are supplied up to 1,500 candle-power, would meet the case; or, failing these, petroleum could be burnt in lamps similar to those used in lighthouses with greater safety, and at not much increased cost, than the compressed-air system. USEFUL NOTES. To ascertain in what direction the electric current is flowing through any wire by means of a pocket compass:— A current flowing from _south_ to _north_ will always deflect the needle to the west, providing the wire in which the current flows is _over_ the instrument. The word S. N. O. W. expresses this—south north over west; and should be remembered. _Another simple plan_ is to hold the outstretched right hand over the compass; then, if the current flows in the direction of the wrist to the fingers, the needle will move towards the thumb. To find the direction of the current in the wire of an electro-magnet: Place the palm of the hand on the coil with the fingers parallel to the wires: the thumb will point to the _North Pole_ if the current is flowing as in previous rule towards the fingers. Conversely: if the _North Pole_ is known, the fingers will point to the direction of the current when placed parallel with the wires, with the thumb pointing to the North Pole.