FINDING TIME FOR BEST LIGHTING.
Suppose, for example, it is wished to ascertain when the .sun will be in the south-west, the time of making the experi ment being, say, 10 a.m. Point the hour hand of the watch to the sun, and mid way between the hour and the twelve, that is to say, facing the eleven, will be the south. Looking upon the watch as a I compass, the north is opposite the five, the east is represented by the eight, and the west by the two. The south-west, therefore, will be midway between the eleven and the two, or at twelve and a half. But this only gives the position of the south-west ; it is required to know at what hour the sun will be in that direc tion. To find this a simple rule must first be committed to memory, namely, that the hour hand goes twice round the watch in the same time that the earth makes one revolution ; or, in other words, the hour hand of the watch moves at just double the rate of the apparent motion of the sun. The application of this is as follows :—The south, in the experiment just made, is represented by the eleven and the south-west by twelve and a half, the distance between these points being, on the watch, hours. Now, it is a
well-known fact that the sun is in the south at 12 noon. Taking, therefore, 12 noon as the starting point, and adding to this twice 11, i.e. 3—because, as just ex plained, the sun moves twice as slowly as the watch—it is readily found that at 3 p.m. the sun will be in the south-west. This useful method of calculation is, how ever, only approximate, and must be em ployed with clue regard to the time of year and the period of sunrise and sunset ; for it is evident that if it is calculated that the sun will be in the east at, say, 6 a.m., while it does not actually rise until 8 a.m., the reckoning will prove misleading and disappointing. A special compass is now obtainable, which shows at a glance at what time the sun will be in any given quarter.