DIAL. 



called the substile : the angle contained 

 between the summit of the stile and the 

 face of the dial is called the elevation. 

 All whidh have their planes, or faces, 

 parallel with the horizon, are called ho- 

 rizontal dials ; those which have perpen- 

 dicular planes or faces, are called verti- 

 cal dials ; and such as are neither vertical 

 nor horizontal, are called reclining dials. 

 When erect dials do not face either the 

 north, or the south, they are called de- 

 clining dials. An universal dial is one 

 that answers for all latitudes. 



The line passing under the centre of 

 the stile longitudinally, and marking the 

 hour of 12, is called the meridian: in de- 

 clining dials, the substile makes an angle 

 with the meridian, in proportion to the 

 deviation from a northerly direction .-this 

 . angle is the difference of longitude. 

 With respect to the manner of construct- 

 ing, and of placing these useful instru- 

 ments, we shall now proceed to give some 

 account. 



The following is the most simple dial 

 that can be made. (Fig. 1. Plate Dialing.) 

 Divide a circle into twenty-four equal 

 parts, and draw, through the several 

 points of division, rays from the centre. 

 That point which is to be the north is to 

 be marked XII, the next on the right 



XI, and thus asfar as V. or IIII : those on 

 the left of the southern point 12, are to 

 be I, II, III, 8cc. in regular order, down 

 to VIII. In the centre, whence the cir- 

 cle was drawn, fix a pin, equal in length 

 to about a diameter of the circle, and be 

 very careful that it be perfectly upright. 

 No\v, placing the dial at such an elevation 

 as may equal the latitude of the place 

 where it is to be used, see that the XHth 

 hour be on the meridional line. Thus, 

 for the latitude of 60, the northerly part 



XII, would require to be raised SO de- 

 grees from the horizon, so that the face 

 of the dial would stand in the plane of 

 the equator, and cause the shadow of the 

 pin to fall on the index, thus to point out 

 the time of day. This is called the equi- 

 noctial dial. 



The following is the best mechanical 

 method known for making common ho- 

 rizontal dials. (Fig. 2.) Draw two con- 

 centric circles, between which the hours 

 are to be marked, and assuming any point 

 for the hour of XII., let the thickness of 

 your gnomon, or stile, be set oft' by two 

 lines a b, c d, passing near the centre e of 

 the circle, perfectly parallel and equidis- 

 tant from the meridional line which pass- 

 es exactly through the centre, and 

 through the mid-day or XII. point. Now 

 cross the meridional line at right angles, 



VOL. IV. 



by the VI. o'clock line fg t and from the 

 points b and d, as centres, describe the 

 quadrants/A,^ i, taking a good extent, 

 so as to approach nearly to the hour cir- 

 cle, for the sake of minute division ; if 

 such be required, divide the quadrants 

 respectively into 90 equal parts; then 

 from d draw a line at 11| from the line 

 e d, which will give the place of I. o'clock ; 

 draw another line at 24$, which will 

 give the place of II. o'clock ; through 



38 T<2 wil1 & ive the P lace of IJI - ' cl ck J 

 through 53^ for IIII. o'clock ; through 

 71-* for V. o'clock; and the line cross- 

 ing your meridian will give VI. o'clock. 

 The base of your gnomon is to be equal 

 to a radius of the inner hour circle, hav- 

 ing the altitude of the place you are at 

 for its angle : thus the gnomon in this fi- 

 gure would have b k, its upper line, 

 standing at an angle of 51^ from its base 

 line a b ; the lowest part being set on to 

 the centre of the VI. o'clock line, even 

 with the meridional line, and the broad- 

 est (i e. highest) part of the gnomon a k, 

 being towards the hour of XII. The left 

 or morning side of the hour table is made 

 by drawing lines, at the angles already 

 described, from the centre/, through its 

 appropriate quadrant. The hours before 

 VI. in the morning, and after VI. in 

 the evening, are adjusted by continu- 

 ing through the centres b and d, re- 

 spectively, those lines which indicate 

 their ascertained numbers : thus the line 

 of V, P.M. continued through the cen- 

 tre d, will give the place of V., A. M. 



This dial must be placed exactly hori- 

 zontal, and its XII. o'clock point, e. the 

 meridional line, must be precisely north- 

 ward. The most certain mode of laying 

 the meridian truly north is by observation 

 of the polar star; or by taking an obser- 

 vation at XII. at noon, by means of a 

 quadrant, when the exact moment of the 

 sun's utmost altitude for the day being 

 ascertained, the shadow thrown upon a 

 plane, by means of a thin cord sustain- 

 ing a heavy plumb, will give a correct 

 northern line, either on the dial plate, 

 already so far prepared on its base, or on 

 any other level surface, from which a 

 true parallel may be taken. With respect 

 to adjusting by aid of a compass, it can- 

 not be recommended : those instruments 

 are often faulty, and when they are not 

 so, the variation is not always exactly 

 laid down. 



To construct a horizontal dial by means 



of a terrestrial globe, elevate the pole to 



the latitude of the place, and bringing 



the nearest meridional line to the brazen 



Y 



