Aug. 24, 1888.] 



SCIENTIFIC NEWS. 



i/i 



SUN-DIALS, AND HOW TO MAKE 

 THEM.— III. 



A VERTICAL wall dial facing due south may be drawn 

 in the same way, but in finding the centre the angle 

 REA, Fig. 8, must be made equal to the latitude of the 

 place. Fig. 11 shows a wall dial and a horizontal dial 



Fig. 11. 



arranged with one style in common to both. It will be 

 noticed that the corresponding hour lines meet at the line 

 where the dials meet. At latitude 45° a wall dial and a 

 horizontal dial would be exactly alike. 



The geometrical construction of a wall-dial which 

 does not face exactly to the south, is rather complicated 

 for those who are not familiar with the art of projection, 



Fig. 1 2. 



but the hour lines can be readily set out with the aid of 

 an equinoctial dial by the following method : — 



Ascertain the true north, and mark the direction on 

 the ground, or on a temporary support beneath the pro- 

 posed dial. This may be done with a compass, making 

 the necessary correction for deviation, and taking care 



that there is" no iron water-pipe near, since this might 

 affect the compass. This line will be the meridian of 

 the place, and is represented in Fig. 1 1 (which is a 

 south dial) by the line CB. Draw a perpendicular line 

 AB. Suppose the place to be near Manchester : we find 



Fig. 13. 



on a map that the latitude is about 53^, and in the table 

 of tangents of co-latitude we find the tangent would be 

 ■738. Divide the line CB into 100 parts; this is easily 

 done by making it 12^ inches long; an eighth of an 

 inch will be the one hundredth part. Make AB equal to 

 73 - 8 of these parts, or nearly o| inches. Join AC by a 

 thread, this will point to the north pole of the heavens, 

 and will be the position of the style, or the edge of the 

 gnomon. 



Now take for an equinoctial dial a circle of card or tin, 



Fig. 14. 

 divided into 24 equal parts, and numbered as in Figs. 

 4 and 5. Hold it so that the thread or wire CA passes 

 through the centre at right angles, and take care that the 

 VI. VI. line is horizontal. Then with a candle or lantern 

 cast a shadow of the thread on to the equinoctial dial 

 and on the wall. When the shadow falls on the XII. 



