Posthuma Fosteri the description of a ruler, upon which is inscribed divers scales: and the uses thereof: invented and written by Mr. Samuel Foster, late professor of astronomie in Gresham-Colledg. By which the most usual propositions in astronomy, navigation, and dialling, are facily performed. Also, a further use of the said scales in deliniating of far declining dials; and of those that decline and recline, three severall wayes. With the deliniating of all horizontall dials, between 30 and 60 gr. of latitude, without drawing any lines but the houres themselves.

SECT. 5.

The Suns place being assigned in any point of the Ecliptick, to finde his Altitude at all houres.

BY this, may Tables of the Suns Altitude be made to all houres, the Sun being in any Signe of the Zodiacke, whereby many particular Instru∣ments for finding the houre of the day, may be made, as Rings, Quadrants, Cylinders▪ and such like.

Draw the line AB, and upon it, with CA or C B, equall to halfe your Scale, describe a Semicircle. Then count upon your Scale the Suns distance from the North Pole, as was done in the 3. Sect. of this Chapt. and in the same manner also finde the summe and difference of this distance and the com∣plememt

[illustration]

of your Latitude. Then take the distance of the said sum and difference in your Compasses, and set it upon the Semicircle from A to D, and draw BD. Now because AB is equall to your whole Scale, you may divide the same into houres, by transferring each 15' gr. from your Scale to the line. This done, take the least distance from the point 1 to the line BD, and set one foot of this di∣stance upon the forenamed difference counted upon the Scale, and let the other foot stand further on∣wards upon the Scale, and where it falls, it sheweth how many degrees that houre of 1 is distant from the Zenith. Or if you count the degrees from the middle of the Scale, it shews the Altitude of the Sun in that Houre. Thus doe for the points of 2, 3, 4, &c. and you shall in the same manner finde their Altitude: And if you go on to the end, you shall (most commonly) finde your Compasses at last to reach beyond the middle of the Scale.

[This alwayes, and then only, happens, when the sum (found at first) is greater than 90 gr.] Look then how much it is beyond, for so many degrees is the Sun below the Horizon at that houre of the night: Or (which is all one) so many degrees is the Sun elevated above the Horizon in that Signe or point of the Eclipticke which is so much on the other part of the Equinoctiall. That is, If the Suns place given were the beginning of Taurus or Virgo, and your Compasses (suppose at the 9^th. houre) goe be∣yond the 90^th gr. of the Scale, you shall there see how low the Sun is under the Horizon at 9 a clock at night, or at 3 in the morning. And the same also sheweth how high the Sun is at 9 in the morning, or at 3 afternoon, if his place were in the entrance of Scorpio or Pisces, which two Signes are so much be∣yond the Equinoctiall on the other part, as Taurus and Virgo are on this side.