9 i

Optics.

The Arm Y X may be of any Length a

the Arm Y Z is of a peculiar Constru , . xten ds no

germinate Length. To this Arm, which ext .farther than ’

w,- 1S adapted a Sliding-Piece % x fharp-

Pomted at 2 . By this the Arm X Z is determined to a| ? n Len gth, the Piece Z x being fixed by the Sciews

, 2 S- Upon this Arm is drawn the short Line vx, byof!? rt , ma y he lengthened in the Whole, and is ^is ?. whole Length X Z when shortest. The Reasoni th 1S Arm is always to increase and decrease.in Pro-P - >on to the Secant of the Sun’s Declination to the

XT whenlhllL b-t th. R»li» » '»‘b.

»f -z- y (the Sen's gr«a«* Decimal,on) as

j 26 -Now the Reason of this Construction of ArmCent 1S \° find ^ any given Day the Distance of thei i\f re of the Speculum S from the Top 1 of t e S y

Su n ’?? ch must ever be equal to the Secant of theLift ^Eelination; for it must always be equa'Jance of the Top of the said Style I from the Centreit R?dL Fork T, indtlu. ta ever equa!

e Lid Secant of Declination. -

pi]: * or since the Style I N and the Fork T are in aParallel to each other, therefore the middleof? m ti* Sides of the Fork being (as they must be)? soe sam e He - ht the End of the Index O as

a ? he .Height of the Style NT, ’«s evident that onth?? noctia l D." o —

^oughi?^ Day the''Sun’s Rays will pass directlyPositio„ 6 Perforation of the Piece R, if it be put in

' .1 -ft Darc»11o1 .1 T'vi r . 1 TT* s • • t .

th.

3 ^Ositio ' * cjrorat ion of the Piece K, it it be putof the Cn P ara Uel to the Plane of the Ecliptic, or thatthe s a j.i p. 0c ,* c '’ 2nd also that the Top of the Shadow of28. T n u- soii exactly on the said Hole.distance ? 18 t^ e Top of the Style is at the least' trom the central Point of R, and therefore

y b e

. J ue r 4- vim. ‘-'i i \<uiu U4V1V<V11'

tl0n above / ? nt ? b y Radius, while in any other Post

Dat-*.: c Os below, . . . mi • J r. '.. ,

Porti

>'kvt,c>n to v the Distance will increase inPro-

JPakc with tft- £ ant of the Angle which the Ray,

... a bv the

-“xe with this first or middle Ray, t a k

1 °P of the Style, and through the Hole p a y

2.9. Now it may be demonstrated, t a ^ ^ xe ftPi the Year, if the Clock and its Pedesta that