Twenty-six important light-houses carry staffs of keepers, but there are 170 lights around the coast, most of them automatic. They go out in the day-time, and their warning beams shine out immediately darkness approaches. Often these lights have their distinctive flashes, for otherwise their identity would not be known. How this remarkable automatic system works we were able to see when the Matai did its first job, replenishing the gas supply for a light on the Hen and Chickens group. The light is nearly 500 feet above sea level and it was the job of the Matai's crew to load a surf-boat with cylinders filled with gas dissolved in acetone, and reduced by immense pressure to liquid state. There is a porous material in the cylinders which eliminates the danger of explosion, a highly essential precaution seeing how these 200 lbs. weights have to be man-handled up a track more fit for goats than sailors. Empty cylinders had to be taken away, the lenses of the light polished, and the delicate mechanism carefully overhauled, so that it could go on doing its duty quite infallibly for four months, if necessary.

These automatic lights depend for their operation on the well-known physical law that light is heat. The governing valve is called a sun-valve, but it is not dependent on what we feel of the warm rays of the sun. What we see is a central rod coated with lamp-black, which absorbs light. Surrounding it are three highly polished rods, light-reflecting. These readily expand in the light, and a lever connects them with the black rod, page 26 which does not expand. On the minute difference in length of these rods, varying with conditions of daylight and darkness, depends the operation of the mechanism which cuts off the gas supply in daylight hours, except for a tiny pilot light, and starts everything up when the light should be showing.