130 



SCIENTIFIC NEWS. 



[Aug. 



hours, the motion being uniformly at the rate of 15 

 degrees an hour. The second motion is that of the earth 

 round the sun. The poles are not at right angles to the 

 path, and, as was explained on p. 373, vol. i., the earth 

 alternately gains and loses part of a revolution, because it 

 does not progress at a uniform speed in its orbit. For 

 this reason the simple sun-dial does not indicate the true 

 time except at the middle of April and June, and at the 

 beginning of September and November. The variation 

 from the true time is given in many almanacks as " sun 

 before," or " sun after the clock." This varies a trifle 

 from year to year, the second year after leap year may 

 be taken as the average. 'A table of this variation will 

 be given further on in this article. 



To an observer on the moon, the earth would form a 

 magnificent timekeeper, hours being marked by the 

 passage of different countries near the Equator into or 

 out of the shadow. Such a dial may be made by setting 

 up a globe so that its axis is parallel to the axis of the 

 earth, as in fig. 1. The north pole of the globe must 

 therefore point to the north pole of the heavens. The 

 Equator being divided into 24 equal parts, the time will 

 be shown by the boundary between the light and the 

 shadow. This line is, however, not well defined, and 

 this renders the method almost useless as a means 

 of determining the time to within a quarter to half an 

 hour, but it is interesting as being the simplest form, as 

 well as that on which all others are based, and in which 

 all others may be said to be contained. Such a globe is 

 sometimes placed on the top of an elaborate dial, and the 

 countries may be represented on it in such a manner 

 that it may be seen at a glance on what part of the globe 

 the sun is shining, and where sunrise and sunset are 

 taking place. The imaginary plane passing perpendi- 

 cularly through such a globe dial,- through its poles, 

 would, if extended, pass also through the poles of the 

 earth. This plane is the meridian of the place, and the 

 line where it cuts the surface of the globe is a line 

 of longitude forming the ". great circle " of the place. In; 

 setting up any dial .it is necessary to find the meridian 

 line. For most purposes this may be done with a good 

 compass, allowance being made for the magnetic variation- 

 or declination of the needle from the true north. This 

 is about 17 degrees to the west at Harwich, 17J at 

 London, 18 at Reading and Peterborough and at places 

 in a line with these towns, 19 at Torquay and Leeds, 

 and 20 at Land's End and Berwick. By drawing 

 these lines on a map, the declination for other places can 

 be found with sufficient accuracy. A large sun-dial can 

 be read to about one minute of time, although one might 

 expect that the edge of the shadow would be too blurred, 

 for the diameter of the sun being about half a degree 

 (see p. 184, first series), and one degree corresponding to 

 four minutes of time, the edge of the shadow is blurred 

 to the extent of about half a degree, or two minutes of 

 time ; it is, however, easy to guess the middle of the in- 

 distinct part. A fine dial in the gardens of Hampton 

 Court is divided into minutes, and may be read to this 

 degree of accuracy without difficulty. This dial is 

 provided with a table of corrections for " sun before " and 

 " sun after clock " calculated to minutes and seconds, and 

 the same information is given in the form of an interest- 

 ing diagram in the centre of the dial. 



The true north can be found by arrangingtwo fine plumb 

 lines in line with the North Star when the constellation 

 of the Great Bear is in either of the positions shown in 

 fig. 2 or in fig. 3. The middle star of the tail being in 



a line with the Pole Star and the true Pole. The stars 

 will be as in fig. 2 on the 10th of October at midnight, 

 10th of November at 10 p.m., 10th of December at 8 p.m., 

 10th January at 6 p.m., and as in fig. 3 on April 10th 

 at midnight, May 12th at 10 p.m., June 10th at 8 p.m., 

 being 3 min. 56 sees, earlier every day. The Pole Star 

 is about 1 degree 18 seconds distant from the true pole. 



The first step towards a practical sun-dial timekeeper 

 is the equatorial or equinoctial dial. This may be 

 supposed to be a thin slice out of a globe dial, the upper 

 and and lower hemispheres being removed, and the 

 axis represented by a thin wire, pointing as before to 

 the North Pole. The circumference of this circle is 

 divided into 24 parts of 15 degrees each, and these may 

 be divided into quarter hours, or into intervals of 

 five minutes, or even minutes, if there be room. As a 

 dial of this kind is very useful in the construction of wall- 

 dials in positions which do not face exactly south, 

 it may be well to remind the reader that it is very easy 

 to divide a circle in twenty-four parts. Draw any circle, 

 and with the same opening of the compasses step round 

 the circumference, thus dividing it into six parts, halve 

 these parts, and then halve again, and these will be the 

 divisions required. The hours may be marked all round 

 so that either side may be used. The XII. line must be 

 set exactly north and south, or in a line with the 

 meridian. The plane of the circle must be set so that 

 it is parallel to the equator of the earth. This is, of 

 course, the case when the axis points to the pole. The 

 angle which the axis makes with the horizon depends on 

 the latitude of the place where the dial is to be used. 

 At the North Pole the dial would be horizontal, and 

 at the Equator it would stand on edge, the axis being 

 horizontal. ' In either' case the VI. line is horizontal, and 

 points east and west. 



The latitude of the place where the dial is to be used 

 must be found on a map, and the plane of the dial must 

 be set at this angle of the horizon. The latitude of 

 London is 5i| degs. The angle may be found by 

 a protractor, or by the use of a table of signs 

 or tangents. The angle should be marked on a 

 card, from which a wedge can be cut, and used for adjust- 

 ing the inclination of the dial with the horizon. Such a 

 dial may be made from a sheet of brass or tin, or even 

 of card, for occasional or indoor use. The axis may be 

 represented by the edge of a piece of metal, as shown in 

 fig. 4. There are in this case really two axes, and two 

 half dials, separated by the thickness of the piece of 

 metal, which is called a style or gnomon. Sometimes 

 a thin wire or thread is used, a support being fixed to 

 hold it, as in fig. 5. In this case, the centre of the shadow 

 must be taken as indicating the time. Many dials are 

 provided with a tolerably thick gnomon, but are divided 

 as though a single axis, such as a fine wire, were used to 

 cast the shadow ; their indications will evidently be inac- 

 curate, especially near noon. 



The equinoctial dial must be divided on both sides, 

 for the sun will sometimes shine on the upper, and 

 sometimes on the lower side. On and near the days of 

 the spring and autumn equinox, hardly any shadow will 

 be thrown, for the sun will be practically in the same 

 plane as the dial. There are many interesting specimens 

 of equinoctial and other dials among the exhibits of 

 mediaeval metal work at the British Museum. These 

 are well worth a visit to any one who is interested in 

 the subject. 



(To be conliitucd.) 



