Of course, the saving of gas which is rendered possible by the use of the sun-valve varies according to the season of the year. During the winter, when the nights are long, the saving may not be very marked, but in the summer, when darkness does not last more than four or five hours, the economy is very noticeable. According to the experience of the Swedish authorities, the average saving of gas during the year varies from 35 to 40 per cent., as compared with similar lights not fitted with this device. But there is another factor which is influenced to a very appreciable degree by the utilization of the sun-valve. By cutting off the light when it is not required, the capacity of--_i.e._, the duration of service upon--one charge is lengthened, and this in the case of an isolated light is a very important consideration. In fact, with the “Aga” system wherein the sun-valve is combined with the flasher, it is possible for the light to work a round twelve months without the least control or necessity for intermediate inspection, and at as low an annual charge as £2 15s., or about $14. [Illustration: THE LAGERHOLMEN LIGHTHOUSE. It marks a lonely dangerous rock in the Baltic Sea, and operates upon the Aga unattended automatic system, with Dalén flasher and “sun-valve.”] One of the latest unattended installations which have been carried out upon these lines is the Lagerholmen lighthouse, marking a dangerous rock in the Baltic Sea. It is a cylindrical tower, with the focal plane 56 feet 4 inches above sea-level, and the flashing light, with sun-valve control, has a range of eighteen miles. The geographical range, however, is only thirteen miles, owing to the comparatively low height of the tower. An interesting and ingenious automatic unattended light has also been established in an isolated part of the Bristol Channel. It was designed by Sir Thomas Matthews, the engineer to the Brethren of Trinity House. This is purely and simply a clockwork-controlled apparatus in which extreme care has been taken to eliminate the disadvantages incidental to such mechanism. This type of light was designed to fulfil three conditions--to give a flashing light; to light up and go out at the proper times; and to require attention only at long intervals. Acetylene is the illuminant used, the gas being stored in a reservoir under high pressure. The gas as it emerges from the supply cylinder is expanded, so that the pressure at the burner does not exceed 2 pounds per square inch.