H O R O 
evening', we may conclude, on the fuppofition of the moon 
being motionlefs, that it is eleven at night. But fince, in 
this interval of 7! hours, the moon has had a retrograde 
motion towards the eaft, which occafions in her paffage 
of the meridian a retardation of 48 m daily ; which is at 
the rate of 2 minutes per hour, we Ihall have, for j\ 
hours, 1 5 m , which mult be added to the hour indicated by 
the moon, over and above the quantity by which her paf¬ 
fage over the meridian has been later than that of the fun. 
If the moon had palled the meridian before the fun, it 
would be necelfary to deduct from the hour indicated by 
the moon the quantity by which Hie preceded the fun, and 
to add to the remainder as many times two minutes as the 
hours Hie indicated. But this calculation, however fliort, 
may be avoided by means of the following fmall machine. 
This machine confilts of two circular plates of brafs or 
wood, or pafte-board, (fig. 20,) one of which AIGH is 
fixed, and the other b ef l moveable. On the fixed plate 
is defcribed a circle a i g h, divided into 24 equal parts, 
reprefenting the 24 hours of the day; each of which muft 
be fubdivided into halves and quarters. Above this piece 
Is applied the other plate b ef l, in fucli a manner as to 
be moveable round the centre C, which is common to 
both; and the circumference of the latter is divided into 
parts which reprefent the hours indicated by the moon on 
a fun-dial. Tliefe hours are not equal to thofe of the fun 
defcribed on the fixed plate, but muft be each 2 minutes 
larger; fince the moon’s daily retardation is about 48 
minutes, or 12 minutes in 6 hours. Therefore, fince the 
degree of a fign is equal to 4 minutes in time, it is evident 
that 3 degrees are equivalent to 12 minutes of time.. For 
this reafon, having drawn the fouth-line ACG, fet off on 
each fide from the point b, to e and l 93 degrees for 6 
hours ; and divide each of thefe fpaces into fix equal parts, 
to reprefent as many hours; then into halves and quar¬ 
ters, as feen in the figure. To ufe this inftrument, place 
the index nb of the moveable piece at the hour of the 
moon’s pafilng the meridian on the propofed day ; then 
obferve the hour indicated by the moon on a horizontal 
fun-dial, and oppofite to the fame hour on the moveable 
piece, you will find on the other, the true hour of the day. 
Dial to Jhow the hour by the Moon. —To employ a dial of 
this kind, it is necelfary to know the moon’s age, which 
may be always found either by a common almanac, or by 
fome of the methods we have already pointed out, under 
the article Astronomy, vol. ii. 
To deferibe a lunar dial on any plane whatever, fuch 
for example as a horizontal one, firft trace out on it a ho¬ 
rizontal fun-dial for the given latitude, and draw the two 
lines 57, 39, parallel to the equinoctial, as fhown in 
Plate V. fig. 21 ; the firft of which being affumed as the 
day of full moon, the fecond will reprefent that of 
new moon, where the lunar hours correfpond with the 
Tolar ; and hence the hour-points marked on thofe two 
parallels, by lines proceeding from the centre of the dial 
A, are common to the fun and the moon. Then divide 
the fpace bounded by the two parallel lines, 3 9, 5 7, into 
12 equal parts ; and through the points of divifion draw 
as many parallel lines, which will reprefent thofe days of 
the moon when ihe lucceffively recedes an hour by her 
own motion towards the eaft, and on which fhe confe- 
quently palfes the meridian every day an hour later. 
The firft parallel, 4 10, being the day on which the moon 
paffes the meridian an hour later than the fun, the point 
B, of 11 hours by the moon, will be the point of noon, 
or 12, according to the fun; as the next, 5 11, reprefents 
a day on which the moon paffes the meridian 2 hours af¬ 
ter the fun, the point C, of 10 hours by the moon, will be 
the point of noon by the fun ; and fo of the reft. 
It is now evident, that if the points 12, B, C, and all 
the others belonging to noon, which can be found by the 
fame method, be joined by a curved line, this curve will 
be the lunar meridian. The other lunar hour-lines may 
be eafily traced out alfo by a fimilar procefs. 
Becaufe the interval between the moon’s co.niun&icn 
Vol X. .No. 662. 
LOGY. 30 f 
with the fun and hef oppofition, that is, between the. time 
of new and full moon, or that when fhe is diametrically 
oppofite to the fun, lo that (lie rifes when the fun fets, is 
about 15 days, all the preceding parallels, except the two 
firft 5 7, 3 9, muft be effaced ; and, inftead of dividing the 
interval into twelve equal parts, it muft be divided into 
fifteen ; in order that you may draw, through the points 
of diviiions other parallels, which will reprefent the days 
of the moon’s age ; and which therefore muft be marked 
with the proper figures along the meridian line, as feen 
in the plate ; by which means the true hour of the night 
may be known, when the moon Ihines, in the following 
manner : In the centre of the dial A, fix an axis or pin, 
fo as to form at that centre with the line A 12 an angle 
equal to the elevation of the pole above the plane of the 
dial, which we fuppofe to be horizontal: this axis, by its 
fliadow on the current day of the moon, will indicate the 
hour as required. 
To find the hour by means of fome of the circumpolar Stars.— 
The hour may be known by a ftar’s palfage by the meri¬ 
dian, or even by its altitude ; for by means of any Ephe- 
meris, and a fliort calculation, we can eafily determine 
how much any ftar precedes or is behind the fun in cul¬ 
minating, or coming to the meridian ; and when this is 
known, together with its declination, the hour may be 
found by obferving its altitude. But a more eafy me¬ 
thod has been invented, by means of a fmall inftrument, 
called the noBurnal. It is adapted for employing the moft 
brilliant of the two laft ftars in the Little Bear, which are 
called its guards. The conftruftion of it is as follows. 
Provide a circular piece of wood or metal, (fig. 22 ;) and, 
having defcribed on it a circle, divide its circumference 
into 365 parts, correlponding to the days of the year; 
which muft be afterwards diftributed into months, accord¬ 
ing to the number that each contains. To this circular 
piece apply another, moveable around the centre, and di¬ 
vide the circumference of it into 24 equal parts, denoting 
the 24 hours of the day. At each of thefe diviiions there 
muft be a fmall notch on the edge, in order that thefe 
parts may be counted in the dark by the touch. One of 
thefe notches however muft be longer than the reft, for a 
purpofe which will be explained hereafter. Then fix to 
the edge of the lower piece a fmall handle ; the middle of 
which ought to be in a line with the centre of the inftru¬ 
ment, paffing through the 7th of November ; becaufe on 
that day the above ftar palfes the meridian at the fame time 
as the fun: that is, above the pole at noon, and below it 
at midnight. Laftly, adapt to the inftrument an index, 
moveable around a pin in the centre; and let a hole be 
pierced in the pin, in order to apply the eye to it. 
To ufe this inftrument, firft make the edge of the long- 
eft notch correfpond with the day of the month ; then ap¬ 
ply your eye to the centre, and, turning towards the north, 
look at the pole-liar, holding the plane of the inftrument 
in a direction as perpendicular as poftible to the vilual ray, 
and the handle of it in the vertical plane ; then move the 
index till the edge of it touches the above ftar, or the 
brighteft of the guards of the Little Bear, and count the 
number of notche. between the index and the longeft 
notch : this number will be that of the hours elapfed af¬ 
ter midnight. The inftrument might be eafily adapted to 
any other ftar: nothing would be neceflaiy but to make 
the fmall handle of the inftrument correfpond with the 
day of the month when that ftar palfes the upper meri¬ 
dian with the fun : in every thing elle the conltrufticn 
would be the fame. 
Dials of a peculiar ConJlruElion. —Every thing hitherto faid 
has related only to the equinoctial and equal hours; fuch 
as thofe by which time is reckoned in England, the day 
being luppofed to begin at midnight, and the hours being 
counted to the following midnight, to the number of 24, 
or twice twelve. This is the moft common method of 
computing the hours in Europe. The aftronomical hours 
are almoft the fame; the only difference is, that the latter 
are counted, to the number of 24, from the noon of one 
4 H day 
