DIALLING. 



713 



Descrip- 

 tion of 

 Dials. 



Universal 

 dial on a 

 cross. 

 Plate 



ccxxx. 



Fig. 11. 



have in the middle of it a groove, to receive a small 

 moveable ring with a hole in it. The divisions K, L, 

 I, A, E, F, G must be marked on the outside of this 

 part of the ring by parallel lines, inscribing on one side 

 the ascending signs, and on the other the descending. 

 When this construction has been made, it will be easy 

 to place the hole of the moveable part A in the proper 

 division ; or at some intermediate point ; for if the 

 ring be pretty large, each sign may be divided into two 

 or three parts. 



To know the hour ; move the hole A to the proper 

 division, according to the sign and degree of the sun's 

 place ; then turn the instrument in such a manner, that 

 the sun's rays passing through the hole may fall on 

 the circle corresponding to the sign in which the sun 

 is : the division on which it falls will shew the hour. 



To render the use of this instrument easier, instead 

 of the divisions of the signs, the days on which the sun 

 enters them might be marked on it. For example, 

 June 21. instead of ss, &c. 



Universal Dial on a Cross. 



90. A dial is said to be universal, when, by adjust- 

 ment, it indicates the hour in any latitude whatevei\ 

 The equinoctial dial described in art. 37. is of this kind, 

 for, from its construction, it can readily be adjusted to 

 any latitude whatever. 



The universal dial on a cross is represented by 

 Fig. 11. It is moveable on a joint C, for elevating it 

 to any given latitude on the quadrant CO go, as it 

 stands upon the horizontal board A. The arms of the 

 cross stand perpendicular to the middle part ; and the 

 top of it, from a to n, is equal in length to either of the 

 arms n e or m L 



Having set the middle line t u to the latitude of your 

 place on the quadrant, the board A level, and the point 

 N northwards by the needle, (allowing for the varia- 

 tion) ; the plane of the cross will be parallel to the 

 plane of the equator ; and the dial will be adjusted. 

 Then from III o'clock in the morning till VI, the up- 

 per edge Ic I of the arm will cast a shadow on the time 

 of the day on the side of the arm c m ; from VI till IX, 

 the lower edge i of the arm i o will cast a shadow on 

 the hours on the side o q. From IX till XII at noon, 

 the edge a b of the top a n, will cast a shadow on the 

 hours on the arm n ef. From XII till III in the af- 

 ternoon, the edge c d of the top part will cast a sha- 

 dow on the hours on the arm klm; from III to VI in 

 the evening, the edge g h will cast a shadow on the 

 hours on the parts p s ; and from VI till IX, the sha- 

 dow of the edge ej will shew the time on the top a n. 



The breadth of each part a b, ef, &c. must be so 

 great as never to let the shadow fall quite without the 

 part or arm on which the hours are marked, when the 

 sun is at his greatest declination from the equator. 

 The shadow will always fall within the arm ef, making 

 its length n e the radius of a circle, the breadth ef ex- 

 ceed the tangent of 23- ° of that circle ; that is, it ought 

 not to be less than about /^ parts of the length : but 

 that the shadow may fall within the quarter divisions 

 of the hours when it comes near the end of the arm, it 

 ought to be almost double that breadth. 



As the cross lies in the plane of the equinoctial, the 

 dials on its arms will be horizontal dials for the equa- 

 tor, and those on its sides east and west dials there. 

 Hence the hour lines may be found as follows : 



Lay the cross on a sheet of paper, and trace its shape, 



VOL. VII. PART II. 



as in Fig. 12, with a black-lead pencil. Then on a, a Descrip. 

 corner of one of the arms with a radius equal to«e, tionol 

 the length of the arms, describe the quadrantal arc ef. _~' a s ' 

 Divide the quadrant into six equal parts at g h i, &c. pTate' 

 and draw lines from a through the points of division CCXXX. 

 to meet the arm in 1, 2, 3, and these are all the hour- Fig. 12. 

 lines that can fall upon it. Divide each of the other 

 arms for the three hours it contains in the same man- 

 ner ; and set the hours to the sides of the arms in their 

 proper places, as marked in Fig. 11. The divisions for Fig. 11, 

 the quarter hours may be found, by dividing the arcs 

 eg, gh, hi into four equal parts. 



An Universal Dial, shewing the Hours by a Terres- 

 trial Globe, and by several Gnomons. 



Universal 

 dial, shew, 

 ing the 

 hours by a 

 terrestrial 

 globe, and 

 by several 

 Gnomons. 

 Plate 

 CCXXX. 

 Fig. 14. 



91. This dial, which seems to have been invented by 

 the very ingenious Mr James Ferguson, may be made 

 of a thick square piece of wood, or hollow metal. The 

 sides are cut into semicircular hollows, in which the 

 hours are placed : The stile of each hollow coming out 

 from the bottom thereof, as far as the ends of the hol- 

 lows project. The corners are cut into angles, in the 

 insides of which the hours are also marked ; and the 

 edge of the end of each side of the angle serves as a 

 stile for casting a shadow on the hours marked on the 

 other side. 



In the middle of the uppermost side or plane there 

 is an equinoctial dial : in the centre whereof an upright 

 wire is fixed, for casting a shadow on the hours of that 

 dial, and supporting a small terrestrial globe on its top. 



The whole dial stands on a pillar, on the middle of 

 a round horizontal board, in which there is a compass 

 and magnetic needle, for placing the meridian stile to- 

 wards the south. The pillar has a joint with a gradua- 

 ted quadrant upon it, (supposed to be hid from sight 

 under the dial in the Figure,) for setting it to the lati- 

 tude of any given place. 



The equator of the globe is divided into 24 equal 

 parts, and the hours are laid down upon it at these 

 parts. The time of the day may be shewn by these 

 hours, when the sun shines upon the globe. 



To rectify and use this dial, set it on a level table, or 

 sole of a window, where the sun shines, placing the 

 meridian stile due south, by means of the compass 

 needle, making allowance for its variation ; or better, 

 by means of a meridian line drawn upon the side of the 

 window. Then bend the pillar in the joint till the axis 

 of the upper dial make with the plane of the horizon 

 an angle equal to the latitude of the place, as measured 

 on the quadrant. When the machine is thus rectified, 

 its plane will be parallel to the equator, and the axis 

 that supports the globe will point to the north pole of 

 the heavens. The same hour will then be shewn in 

 several of the hollows, by the ends of the shadows of 

 their several stiles. The shadow of the axis of the 

 globe will shew the hour on the equinoctial dial, from 

 the 20th of March to the 22d of September ; and if the 

 meridian of the place on the globe be set even with the 

 meridian stile, all the parts of the globe that the sun 

 shines upon will answer to those places of the real earth 

 which are then enlightened by the sun. And if the 

 hour of VI be marked on the equator in the meridian of 

 your place, the division of the light and shade on the 

 globe will shew the time of the day. 



The construction of this dial is as follows: on a 

 thick square piece of wood, or metal, draw the lines 

 a c and b d (Bg. 14.) as far from each other as you in- Fig. 14. 

 4x 



