DIAL. 



meridian, set the index to XII. upper- 

 most. Now turn the globe until the in- 

 dex comes exactly to I. which will be 

 effected by a movement of 15 measured 

 on the equator; each hour making a 

 change of 15 on the sun's place thereon; 

 (for 24 hours multiplied by 15 make a 

 total of 360, equal to a whole circle.) 

 The place where the meridional line 

 moves to should be marked, for that will 

 give the place of I. o'clock; move on 

 15 more, to find where the meridional 

 line cms the equator, for the place of II. 

 o'clock; and thus in succession will all 

 the hours' places be indicated by the 

 meridian at each change of 15 on the 

 horizon ; always measuring close to the 

 meridian, or following the hour hand on 

 the index. If the latter be marked with 

 halves and quarters of hours, the corres- 

 ponding divisions may be made on the 

 equator, as the meridian by which you 

 are governed passes on, taking 7\ for a 

 half hour, and 3 c for a quarter ; but 

 such is seldom required, except in very 

 large dials; the eye usually judging, with 

 tolerable precision, of the quantity cover- 

 ed by the shade of the gnomon. 



To set this off on paper, measure the 

 number of degrees between the several 

 parts of the meridian, noted down as 

 stations of the governing meridional line, 

 when passed on 15 at each movement. 

 This being done from XII. to VI. will 

 establish a measurement that may be 

 imitated on any scale, observing to draw 

 the VI. o'clock line correctly, and to 

 give a proper length of base, as well as 

 a due latitudinal angle to the gnomon. 

 The method suggested by Mr. Ferguson, 

 of filling up the interior ot the horizon 

 after drawing- forth the globe, is prolix, 

 and suits but few persons: whereas, the 

 knowledge of how many degrees are in- 

 cluded in each interval, respectively, 

 proves 1 a sufficient guide. Those inter- 

 vals will be found to correspond with 

 what have been directed in describing 

 the second figure. 



To make an erect dial directly south. 

 Fig. 3. On this the sun can shine only 

 from VI. A. M. to VI. P M. ; therefore, 

 only the intermediate hours need be 

 noted. Elevate the pole as before di- 

 rected, bnt, in lieu of placing XII. to the 

 north, place it to the south of your dial 

 plate, which in this kind is most conveni- 

 ently made an obiong, projected verti- 

 cally below the VI. o'clock line. Pro- 

 ceed to find the places of the hours as 

 before shewn, trace them through their 



centres to the opposite parts of the hour 

 circle, for on that they are to stand. The 

 gnomon is to make an angle equal only 

 to the co-latitude of the place: thus, if 

 you are in 51-, the co-latitude (being 

 the amount required to complete to 90) 

 would be 38. 



Or you may proceed as follows : on the 

 meridional line a b t as it points down- 

 wards from the foot, or lowest part of 

 the gnomon, measure off at pleasure any 

 distance, as a c, for the size of your dial ; 

 at c erect the perpendicular C D, and 

 make the angle CAD equal to the eleva- 

 tion of the equator; then make a second 

 triangle CD E, the angle at D being equal 

 to that at A. Through E draw G H at 

 right angles with A E. Carry on E B 

 equal to E D, and with that distance as a 

 radius, describe the quadrant E F from 

 B as a centre. Measure off the proper 

 angles from the point B, through the se- 

 veral parts of the quadrant, which is di- 

 vided off into six equal parts ; these will 

 fall upon the prolonged line G H, and 

 give points thereon, through which lines 

 being drawn from the centre A of the VI. 

 o'clock line to the hour frame, the places 

 of the several hours will be given. The 

 gnomon is fixed at A, equal to A D E ; 

 being conformable to the co-latitude ; or 

 it may be simply a wire fixed at C, equal 

 in length to C D, but perpendicular to 

 the face of the dial ; some use large an- 

 gular iron rods. This kind of dial is often 

 seen on the sides of country church- 

 steeples facing the south. 



To make an erect dial facing the north. 

 Invert the whole of that just described, 

 making the gnomon point upwards in- 

 stead of down wards, and causing all the 

 lower points to be transferred from left 

 to right, and from right to left. This 

 kind of dial will shew the hours before 

 VI. A. M. and after VI. P. M. When such 

 is wanted, the best way is to set up a 

 stout post, with the planes of two dials 

 back to back, they pointing due south 

 and due north, respectively ; thus, as the 

 pin retires from one, it will set upon the 

 other. 



We shall now instruct the reader 

 how to make those scales, which are 

 indispensable towards the attainment of 

 perfection in this pleasing branch of 

 study. 



Tiie lines useful in dialling ave, 1, a 

 line of chords ; 2 a line of latitudes ; 3, 

 a line of sines ; and 4, a line of hours. 

 They are all derived from the quadrant 

 of a circle, as will be shewn in fig. 4. 



