CHAP. 74. (72.)—REMARKS ON DIALS, AS CONNECTED WITH
THIS SUBJECT.
The same dial-plates[1] cannot be used in all places, the
shadow of the sun being sensibly different at distances of
300, or at most of 500 stadia[2]. Hence the shadow of the dial-pin,
which is termed the gnomon, at noon and at the summer
solstice, in Egypt, is a little more than half the length of the
gnomon itself At the city of Rome it is only 1/9 less than
the gnomon, at Ancona not more than 1/35 less, while in
the part of Italy which is called Venetia, at the same hour,
the shadow is equal to the length of the gnomon[3].
1. "Vasa horoscopica." "Vasa horoscopica appellat horologia in plano
descripta, horizonti ad libellam respondentia. Vasa dicuntur, quod area
in qua lineæ ducebantur, labri interdum instar et conchæ erat, cujus in
margine describebantur horæ. Horoscopa, ab w(/ra et
skope/w, hoc est,
ab inspiciendis horis." Hardouin, in Lemaire, i. 391.
2. These distances are respectively about 38 and 62 miles.
3. We are not to expect any great accuracy in these estimates, and we
accordingly find, that our author, when referring to the subject in his
6th book, ch. 39, makes the shadow at Ancona 1/35 greater than the gnomon,
while, in Venetia, which is more northerly, he says, as in the present
chapter, that the shadow and the gnomon are equal in length. See the
remarks of M. Alexandre in Lemaire, ut supra.