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How to Make a Sun-Dial - from Harper's

 

Our young friends would, we doubt not, like to know how to make a sun-dial that will give the time very accurately. Common sun-dials depend on the shadow of a post, which is thick and heavy, and affords only a very rough idea of the time. But the one we are going to tell them about will show the time as precisely as a clock. And it is quite easy to make. It has, in the first place, a face set up slanting on a pedestal. The proper slant answers to the latitude of the place. At and near New York it should be about forty-one degrees from the perpendicular, or a little more than half upright. The face is divided into hour spaces, just like the face of a clock, but the whole circle is not used. A semicircle is all that the sun can traverse, except in the long days of summer. The fourth part of a circle is about all that can be used in ordinary windows. It will answer for the hours between nine o'clock and three. It is divided into six equal parts for the hour spaces, and each of these is subdivided for the minutes. If the radius of the circle be one foot, the minute spaces will be about one-sixteenth of an inch, or about the same as on the face of a watch. The dividing is easily done with a pair of compasses, a ruler, and a sharp lead-pencil.

Now we will explain the indicator. It is made of three pieces—a base and two uprights. The base is fifteen inches long, three wide, and three-quarters of an inch thick. The uprights are of the same thickness, and about seven inches high. They are morticed into the base, and have the shape shown in the picture. A hole half an inch in diameter is bored through the upright at A, and another at B. Over each of these holes pieces of tin are tacked, with a little hole in the centre about as large as a pin's head. When the sun-dial is placed in position, the sun shines through these holes, and makes a little bright circle on the other upright. The upper hole, A, is for summer, when the sun is high, and the lower one, B, for winter. The indicator is pivoted by a large screw to the centre, C, of the face, so that it can be turned round like the hand of a clock. At the upper end of the indicator a little pointer is fastened directly over the scale of hours and minutes. A needle, or a pin with the head cut off, makes a good pointer.

After the sun-dial is made, the next thing is to set it in its proper position, which is so that when the pointer is at XII. it will also be directed exactly south, while the lower end of the indicator is to the north. Then, at noon by sun time, the sun will make its little bright circle exactly in the middle of the lower upright. A line should be drawn up and down to show the middle; then this line will cut the sun circle equally in two. To find out the time before and after noon, the indicator is moved so that the sun circle will fall on the same middle line, and the pointer will show the time. This sun time differs somewhat from clock time. The difference for every day in the year is given by the almanacs, and very exactly by the Nautical Almanac. This difference being added or subtracted, makes known the true clock time. Thus, for the 1st of March, clock time is twelve minutes faster than sun time. Hence noon by the sun-dial is just that much later than noon by the clock. Any of our readers who have a little mechanical skill can make a sun-dial, on the plan described, that, when put in proper position, will be more reliable than the best of clocks, and that will be found a convenient means of setting them right. But don't despise the clocks; for very likely you will have to resort to one in order to get the sun-dial in position; and then, too, remember that the sun does not shine all the while, but is very fond of hiding behind clouds.