DIAL 
-deaves, and the Lean one of 15, both which confequently 
e fame period of one hour; the under 
tube were not interpoled ; the period of this large wheel is 
gal ofa folar hour, or 6" 5" 15°, and it carries round with it 
he loweft {mall face in fiz. 2, in a direGtion contrary to that 
of the motion of the feconds’ hand, which points to its divi- 
‘fions, and alfo to the fixed divificns of the circle furrounding 
this fmall face on the large dial itfelf. Again, the upper 
cannon pinion, of 15 leaves, a€tuates one of the two wheels 
of 75 teeth that revolve on the fufee’s arbor, as on a ftud, 
at the point d, both of which — being of the fame dia- 
meter and conftruction, and lying 
to an eye placed over them in a oa as 
wheel, however, in queftion, 1 is the under one, to which a pi- 
nion of 15, not feen, is attached, that drives the under one 
of two finilar one of 72 tah each, round a ftud at the 
B 
d = 
point 4, in the perio xe Z ~ ofa folar hour, that is, in 24 
hours of folar time; aac upon rde el of 72 
is the folar hour hand placed, a oa in an ae oa 
day, and indicates the 24 hours of mean folar time on the lar 
circle of Roman characters, that furrounds the left-hand fmall 
face. Thus, the three hands indicate the folar hours, minutes, 
and feconds on the three fixed circles of the large plate or 
dial. We have already feen, that the great wheel of 487 
teeth revolves, byits connection with the cannon pinion, of 80, 
, 487 
of an hour, or in 6" 5" 15°, which period is exa@ly 
a minutes of folar time; to this wheel a pinion of 12 
leaves is faft, that impels a wheel of 72 teeth round a ftud, 
or pin, on the cock f, that lies over a part of wheel 487, 
at the point e; this wheel has again a pinion faft to it of 8 
leaves, that impels the wheel of 80 teeth round the cannon 
: @ 80 6c eas ee 
at a,in — =e = of 3654 minutes, which period is 
rs? 5 or 365% hours; and to the tube of this wheel 
of 80 ce eee right-hand {mall plate, or dial, is at- 
tached by fri€tion, and, confequently, revolves with it, as 
the {mall dial on the inferior part of the face docs with the 
large wheel of 487, to which it is alfo attached by the fric- 
: 7 7 Gu. 
tion of their tubes; and, laftly, a train of i 4 = in every 
I 
refpe& fimilar to the one of this denomination, aa de- 
cri oo converts the 3654 hours into 3 » for the 
the third, or left-hand {mall dial’s retrograde revo- 
j " that i is, the pinion of 15, faft to the w of 80 
teeth, drives the upper 75 round the point d, a ps pinion 
of 15 again drives the upper 72 in the manner before de- 
Now, 
the manner in which fidereal time is indicated is, by means of 
the folar hands pointing to the three {mall dials that ue 
in a retrograde direGion, as they regard thefe hands, in 365% 
. YS, 3652 hours, and 365% minutes re{peGtively ; ; the fidereal 
hours, minutes, and feconds bein orter in durati on than 
folar ones, by fuch a minute quantity, as amounts, in the ag- 
weregate, to an entire fidereal day in a year, conftitute that 
{cribed, the train being, in both cafes, equal to ~4 
{pecies of time which is ufed as the meafure of the right 
afcenfion of any of the heavenly bodies, and which is itfelf 
meafured by the earth’s abfoiute rotations on its axis, as 
they have a reference to a ftar, or any fixed point in the 
h 
motion 3 
earth’s annual progrefs in the ecliptic. 
tween a folar and fidereal day, in folar time, is fomewhat 
» fo that a diffcrence, or 
in about fix minutes of folar 
somes 
time, namely, in 
= 67.0876; hence, fuppofing the figures of 
the fall dials to be put, at any time, to coincide ay oa 
m n i 
d face, and Edereal 
t ahi: Gage we call the 
fidereal feconds? dial, and the d: Hetence of the two indica- 
tions two fec n hke e manner, after fixty fuch 
periods of fix minutes scaly, that is, after upwards of fix 
hours, one minute-{pace of difference will be indicated by 
the minutes’-hand, when the time, fhewn on the moveable 
dial, is compared with the time on the fixed circle of 
minutes, and the feconds’-hand will now, as at firft, indicate 
both folar and fidereal feconds alike, by reafon of the firlt 
minute of difference being com leted ; and thus, in the 
courfe of the whole 24 folar hours, there will be a difference 
of nearly four minute fpaces fhewn by the minute hand ; 
but as yet there is {carcely any alteration perceptible in the 
hour-circles, nor will there be a difference of one hour’s 
fpace till upwards of 15 days have tranfpired. Stridly 
{peaking, the period in which the eee will amount 
to a fecond, by the wheel-work, will be a7 of a minute, or 
6.0875 minutes, and as the anoual period of the retrograde 
wane of the fidereal hour-dial is 365° 6" 0”, inflead of 
e4 6" o™ xr5.2, the whole error i the indication of 
Baoreal time, by the folar hands pointing to the fidereal, 
or moveable dials, will be little more than one fecond an 
half in a fidereal year, which, it will be allowed, is hardly 
worth naming. The adjuftment of the dials is performed 
by the fun’s mean right afcenfion, which is determined from 
te apparent right afcenfion, given noon of each day, 
n the Nautical Almanac, and in ’s Ephemeris, ‘e 
applying the equation of time, ae wih the accelera- 
tion added thereto, at the rate of one fecond in each fix 
minutes of equation, to convert it into fidereal time ; but care 
muft be taken to apply the equation with a contrary fign, to 
gr ae ae 2 < 7 
When the fun’s mean right afcenfion is thus obtained in 
hours, minutes, and feconds, at the noon y day, the 
plate, in which fituation of things, 
dicate not only folar and fidereal cae but alfo the fun’s 
mean right afcenfion at mean noon, from which the ap- 
; 6 parent 
