Lectures on select subjects in mechanics, hydrostatics, pneumatics, and optics. With the use of the globes, the art of dialing, and the calculation of the mean times of new and full moons and eclipses / By James Ferguson.

  • Ferguson, James, 1710-1776.
Date:
1760
Catalogue details

Licence: Public Domain Mark

Credit: Lectures on select subjects in mechanics, hydrostatics, pneumatics, and optics. With the use of the globes, the art of dialing, and the calculation of the mean times of new and full moons and eclipses / By James Ferguson. Source: Wellcome Collection.

    451/486 (page 395)
    395 Of dialing. dial, from the center on either fide of the fubftlle CSK, and the correfponding co-tangents of altitude from the fame center for I, II, III, &c. hours before or after the fun is in the horizon of the place for which the diaj is to ferve, on the equi-* nodlial and folftitial days. 2. One, of thefe dials has its name ,from the hours being reckoned from fun-rifing, the begin¬ ning of the Bahylo'riian day. But we are not thence to imagine that the equal hours, which it Ibews, to be thofe in which the aftronomers of that coun¬ try marked their obfervations. Thefe, we know with certainty, v/ere unequal, like the aa being twelfth parts of the natural day : and an hour of the night was, in like manner, a twelfth part of the night; longer or fhorter according to the feafon of the year. So that an hour of the day, and an hour of the night, at the fame place, would ^ always make yt 24, or 2 equinodlial hours. In Paleiline, among the Romans, and in feveral other countries, 3 of thefe unequal nodlurnal hours were a vigilia or watch. And the redudlion of equal . and unequal hours into one another, is extremely I i eafy. If, for inftance, it is found, by a foregoing I rule, that in a certain latitude, at'a given time of I , the year, the length of a day is 14 equinodlial ! i hours, the unequal hour is then or of an hour, i j that is, 70 minutes *, and the nocturnal hour is 50 j minutes. The firft watch begins atVII (funTet), the * j fecond at three times 50 minutes after, viz. IX h. , 30 m. the third always at midnight *, the morning ^ W^tch at J hour pad Ik If