712 



DIALLING. 



Descrip- 

 tion of 

 Dials. 



Dial on a 

 cj liiider. 



Plate 



ecxxx. 



Fig. 6, 7. 



King dial 



must be parallel to AC, and as far from it as is equal to 

 the intended circumference of the cylinder. 



3. Divide the space between AC and BD (at the top 

 and bottom) into 12 equal parts, for the signs of the 

 ecliptic, and from mark to mark of these divisions draw 

 lines, which will be parallel to AC and BD, and place 

 the characters of the twelve signs on these twelve spaces 

 at the bottom, as in the Figure. The spaces between 

 the signs may be subdivided into halves, and, if there 

 be room, into quarters. 



4. At the top of the dial, make a scale for the months 

 and days of the year, so as the days may stand over the 

 sun's place for each of them, in the signs of the ecliptic. 

 The sun's place must be taken from an ephemeris. 



5. Compute the sun's altitude for every hour, in the 

 latitude of your place, when it enters each sign of the 

 ecliptic, and also when it is in the middle of the sign, 

 and in the upright parallel lines at the beginning and 

 middle of each sign, make marks for those computed 

 altitudes among the horizontal parallels of altitude, 

 reckoning them downwards, according to the order of 

 the numeral figures set to them at the right hand, an- 

 swering to the like division of the quadrant at the left, 

 and through these marks draw the curve hour lines, 

 and set the hours to them, as in the Figure, reckoning 

 the forenoon hours downward, and the afternoon hours 

 upward. The sun's altitude should also be computed 

 for the half hours ; and the quarter hour lines may be 

 drawn very nearly in their proper places by estima- 

 tion and accuracy of the eye. Then cut off the paper 

 at the left hand close by the line AC, and also the paper 

 on the right hand close by the line BD, and cut it also 

 close by the top and bottom horizontal lines, and it will 

 be fit for pasting round the cylinder. 



This cylinder (Fig. 7.) should be hollow, to hold 

 the stile DE when it is not used. The crooked end 

 of the stile is put into a hole in the top AD of the cy- 

 linder, and the top goes on tightish, but must be made 

 to turn round on the cylinder like the lid of a paper 

 snuff box. The stile must stand straight out, perpen- 

 dicular to the side of the cylinder, just over the right 

 line AB, where the parallels of the sun's altitude begin ; 

 and the length of the stile, Or distance of its point c 

 from the cylinder, must be equal to the radius a A of 

 the quadrant. (Fig. 7-) 



91. To use this dial, place the horizontal base BC of 

 the cylinder on a level table, where the sun shines, and 

 tarn the top AD, till the stile stands just over the clay 

 of the then present month. Then turn the cylinder 

 about on the table, till the shadow of the 6tile falls on 

 it, parallel to those upright lines which divide the signs; 

 that is, till the shadow be parallel to a supposed axis in 

 the middle of the cylinder, and then the point, or low- 

 est shadow, will fall upon the time of the day as it is 

 before noon or afternoon, among the curve hour lines, 

 and will shew the sun's altitude at that time among the 

 cross parallels of his altitude, which go round the cylin- 

 der, and at the same time it will indicate the sign of 

 the ecliptic in which the sun is. The degree of the 

 sign may be estimated nearly by the eye. 



The dial may also be suspended by the ring F at the 

 top, and when it is not used, the stile may be drawn 

 out, and put into the cylinder. 



Ring Dial. 



<)1. Ring dials are another variety of the kind that 

 indicates the hour by the sun's altitude. They are 

 very common ; but those generally sold are inaccurate 

 in the principle of their construction, for the hours are 



usually marked in the inside on one line, and a narrow 

 moveable band, with a hole in it, is shifted till the hole 

 correspond with the degree and sign of the sun's place 

 marked on the outside. Instead of one circle, there 

 ought to be seven distinct circles on the concave sur- 

 face of the ring, to represent as many parallels of the 

 sun's entrance into the signs ; and on each, there must 

 be marked the sun's altitude on its entrance into the 

 sign belonging to the parallel to which the circle cor- 

 responds. When these points are marked, they must 

 be joined by curves, which will be the real hour lines. Plate 



Having provided a ring (Fig. 8.), or rather descri- S^*** - 

 bed a circle of the same size as the ring which is to be "S* 8 * 

 divided, and having fixed on B as the point of suspen- 

 sion, make BA and BO, on each side of B, equal to the 

 latitude of the place ; that is, equal to the distance of 

 the zenith from the equator. Then through the points 

 A and O draw the chord AO, and AD perpendicular 

 to it : If the line A 12 be then drawn through A, and 

 the centre of the circle, the point 12 will be the hour 

 of noon on the day of the equinox. 



To find the other hour-points for the same day at 

 the commencement of Aries and Libra ; from the centre 

 A describe the quadrant OD ; and from O set off to- 

 wards P, the sun's altitude at different hours of the 

 day; as at 1 and 11, 2 and 10, &c. The lines drawn 

 from the centre A through these points of division, if 

 continued to the circumference of the circle B 12 A, 

 will give the hour-points of the day of the equinox. 



To obtain the hour-divisions on the circles corre- 

 sponding to the other signs ; first set off on both sides 

 of the point A (Fig. 9-), the sun's declination when Fig 9. 

 he enters each of the signs, viz. the arcs AE and AI 

 of 1 1° 30' for the commencement of Taurus or Virgo, of 

 Scorpio or Pisces; AF of 20° 13' for the commence- 

 ment of Gemini and Leo ; AK equal to it for the com- 

 mencement of Sagittarius and Aquarius ; and AG and 

 AL of 23° 30' for the commencement of Cancer and Ca- 

 pricorn. 



Now, to find the hour-points on the circle, that cor- 

 responding to the commencement of Aquarius, for ex- 

 ample ; through the point K, which corresponds to the 

 sun's entrance into that sign, draw KP parallel to AO, 

 and also the line K 12 : From the same point K describe 

 between K 12, and the horizontal line KP, the arc QR, 

 on which set' off from R towards Q, the sun's altitude at 

 the different hours of the day when he enters Sagitta- 

 rius and Aquarius, as seen in the Figure ; and if lines 

 be drawn from K to these points of division, you will 

 have the hour-points of the two circles corresponding 

 to the commencement of Sagittarius and Aquarius. By 

 proceeding in the same manner for the sun's entrance 

 into the other signs, you will have the hour-points in 

 the circles which correspond to them. 



Then trace out on the concave surface of the circles 

 seven parallel circles (Fig. 10.), that in the middle for 

 the equinoxes ; the two next on each side for the com- 

 mencement of Taurus and Virgo, Scorpio and Pisces ; 

 the following two on the right and left for Gemini and 

 Leo, Sagittarius and Aquarius ; and the last two for 

 Cancer and Capricorn : If the similar hour-points be 

 joined by a curve line, the ring dial will be completed. 

 The next thing to be done, is to adjust properly the 

 hole which admits the solar rays ; for it ought to be 

 moveable, so that on the day of the equinox it may 

 be at the point A ; on the day of the summer solstice 

 at G ; on the day of the winter solstice at L ; and on 

 the other days of the year in the intermediate positions. 

 For this purpose, the exterior part of the ring must 



5 



Fig. 10. 



