S02 H O R O 
day to the noon of the clay following. But there are fome 
other kinds of hours, which it is proper we fhould here 
explain; becaufe they are fometimes traced out on fun- 
dials: fuch are the natural or Jewifh hours, the Babylo¬ 
nian, the modern Italian, and thofe of Nuremberg. 
The natural or Jewifli hours begin at fun-rife 5 and 
there are reckoned to be u between that period and fun- 
fet: hence it is evident that they are not of equal length, 
except on the day of the equinox : at every other time of 
the year they are unequal. Thofe of the day, in our he- 
mifphere, are longer from the vernal to the autumnal equi¬ 
nox : thofe of the night are, on the other hand, longer while 
the fun is palling through the other half of the zodiac. 
The Babylonian hours were of equal length, and began 
at fun-rife; they were counted, to the number of 24, to 
lun-rife of the day following. The modern Italian hours 
(for the ancient Romans counted nearly as we do from 
midnight to midnight) are reckoned to the number of 24, 
•from l’un-fet to fun-fet of the day following; fo that on 
the days of the equinox noon takes place at the 18 th hour, 
and then, as the days lengthen, the aftronomical noon 
happens at 17J hours, then at 17 hours, &c. and vice verfa. 
See, however, p. 292. 
To trace out, on a Dial, the Italian Hours. —Defcribe firft 
on the propofed plane, which we here fuppofe to be a 
horizontal one, a common horizontal dial, with the aftro- 
nomical or European hours: delineate on it alfo the arcs 
of the folllitial iigns, Cancer and Capricorn; as well as 
the equinoctial line, whic-h is the arc of the equirio&ial 
figns. Then obferve that, on the days of the equinox, 
noon, for a dial conftru&ed at London, takes place at 
the end of the 18th Italian hour; and on the day of the 
fummer folftice at 17 minutes after the 16th hour. Noon, 
therefore, or 12 hours, counted according to the aftrono- 
mical hours, correfponds, on the day of the equinox, to 
the 18th Italian hour;, and on the day of the folftice to 
17 jninutes after the 16th; .confequently the 18th Italian 
hour, on the day of the fummer folftice, will correfpond 
to 17 minutes paft 2, counted aftronomically. Join there¬ 
fore, (fig. 23,) by a ftraight line, the point of noon marked 
on the equinoctial line, and that of 2 hours 17 minutes on 
the tropic or arc of the fign Cancer, and inferibe there 18 
hours. Join alfo by tranfverfal lines 1 hour on the equinoc¬ 
tial and 3 h 17™ on the arc of Cancer; then 2 h and 4 h i7 m , 
See. and before noon 1 n> and i h i7 m ; io h and n h i7 m ; 
9 h and n h 17™, See. efface then the aftronomical hours', 
which we fuppofe ought not to appear, and continue the 
above tranfverfal lines till they meet the parallel of Capri¬ 
corn, inferibing at their extremities the proper numbers; 
by which means you will have your dial traced out as 
feen in the figure. 
It may be eafdy feen, by the above example, what cal¬ 
culation will be necelfary for a latitude different from 
that of London, where the length of the day, at the fum¬ 
mer folftice,’is 16 hours 34 minutes, and at the w'inter 
folftice only 7 hours 44 minutes. In another latitude, 
where the longeft day is only 14 hours and the fhorteft 10, 
noon at the fummer folftice will take place at the end of 
the 17th Italian hour. Noon therefor •, or 12 hours, count¬ 
ed aftronomically, will on the day of the folftice correfpond 
to the 17th Italian hour; and confequently the 18th Ita¬ 
lian hour, at the fame period, will correfpond to 1 in the 
afternoon counted aftronomically. To have the hour-line 
of the 17th Italian hour, therefore, nothing will be ne- 
ceffary, but to join the point of 1 in the afternoon, on the 
arc of Cancer, and the point of noon on the equinoctial. 
And the cafe will be the fame with the other hours. 
To trace out on a Dial the Lines of the Natural or JewiJh 
Hours. —We have already faid, that the equal hours which 
can be counted from fun-rife to fun-fet, to the number of 
12, are called the natural hours; for it is this interval of 
time which really forms the day. 
This kind of hours may be eafdy traced out on a dial, 
which we Hull here fuppofe to be horizontal. For this 
LOGY. 
purpofe, it will be firft neceffary to draw the equinoflial 
and the two tropics by the preceding methods. Now it 
muft be obferved, that as, in the latitude of London, the 
fun, on the day of the fummer folftice, rifes at 3 h 43and 
fets at 8 h i7 m , the interval between thefe periods is equal 
to 17 11 34 m ; confequently, if we divide this duration into 
12 parts, each of thefe will be about if hour: for this 
reafon, draw lines from the centre of the dial to the points 
of divifion on the equinoflial, correfponding to j| hours, 
to 7 hours, to 8| hours, to 10 hours, to nj hours, fo 
1 hour, and fo on; but marking only, on the tropic of 
Cancer, the points of interfeflion which thefe hours form 
with it. In like manner, as the fun at the winter folftice, 
in the latitude of London, rifes at 81 1 8 m , and fets at 
3 h 52“ the duration of the day is only 7 hours 44 minutes ; 
which being divided into 12 parts, gives for each about 
40 minutes, or J of an aftronomical hour. Draw there¬ 
fore the hour-lines correfponding to 8§ hours, to 9^ hours, 
to 10 hours, and fo on; marking only the points where 
they interfefl the tropic of Capricorn; then, if the cor¬ 
refponding points of divifion, ontfhe two tropics and the 
equinoflial,' be joined by a curved line, the dial will be 
deferibed, as fhown at fig. 24. 
If more exactnefsbe required, it will be neceffary to trace 
out two more parallels of the figns, viz. thofe of Taurus 
and Scorpio, and to find on each, by a fimilar procefs, the 
points correfponding to the natural hours: the natural 
hour-lines may then be made to pafs through five points,by 
which means they will be obtained with more exaflnefs. 
Of fixing Dials .—Every dial muft be fo placed, that the 
upper edge of the ftyle may point direftly to the pole, and 
that the horizontal line be perfeftly level, and that it hav# 
its proper declination and inclination; and in an uptight 
dial, that the 12-o’clock line be perpendicular to the ho¬ 
rizon. Having prepared the plane on which the dial is 
to reft, the hour may be found with tolerable accuracy by 
a large equinoctial ring-dial, to which a watch is to be 
fet; and then the dial may be fixed by the watch at lei¬ 
sure. But, in order to have the time more exa&ly, the fun’s 
altitude fhould be taken by a good quadrant, and the pre- 
cife time of obfervation noted by a dock or watch: then 
the time fhould be computed for the obferved altitude, 
and the watch fet to agree with that time. Otherwiie, 
take two equal altitudes of the fun in the fame day in 
fummer; one between feven and ten in the morning, the 
other between two and five in the afternoon; mark the 
moment of the two obfervations by a clock or watch ; and, 
if the watch fhows the obfervations to be at equal diftances 
from noon, it agrees exa&ly with the lun; if not, the 
watch muft be corrected by half the difference of the fore¬ 
noon and afternoon intervals 5 and then the dial may be 
fet true by the watch. 
It is neceffary to obferve, that if a dial be made accord¬ 
ing to-the ftrift rules of calculation, and truly fet at the 
inlfant w'hen the fun is in the meridian, it will be a mi¬ 
nute too faft in the forenoon, and as much too flow in the 
afternoon, by the thadovv of the ftyle; becaufe the edge of 
the fhadow that fhows the time is even with the foremoft 
edge of the fun in the forenoon, and with his hindmoft 
edge in the afternoon, on the dial, whereas the fun’s cen¬ 
tre determines the time in the (fuppofed) hour-circles of 
the heaven; and, as the fun is half a degree in breadth, 
he takes two minutes to move through a fpace equal-to 
his breadth; and there will be two minutes at noon in 
which the fhadow W'ill have no motion at all on the dial. 
It likevvife appears, that, if the dial be fet true by the fun 
in the forenoon, it will be two minutes too flow in the 
afternoon ; and vice verfa. The way of remedying this 
error, is to fet every hour and minute divifion on the 
dial one minute nearer XII than the calculation requires. 
Every dial is alfo too faft in the forenoon and too flow in 
the afternoon, on account of the refraction of the fun-, 
beams, by which the fun is railed higher, and the fhadovy 
brought nearer the fubftyle. 
3 H. Of 
