VOL. III.] PHILOSOPHICAL TRANSACTIONS. 325 



II. Ephemerides Mediceorum Syderum, ex Hypothesibus et Tabulis Job. 

 Dom. Cassini. Bononiae, 1668, in thin fol. 



What Gahlaeo Galilaei undertook, after he had discovered the satellites of 

 Jupiter, of giving an easy and sure way to know the longitudes by a careful ob- 

 servation of those stars ; Signior Cassini seems to have now performed more 

 fully than others, by composing certain tables, after 15 years observations 

 made with exactness of the motion of the said satellites. These tables are con 

 tained in this book ; and for verifying them, he has added the Ephemerides 

 of those stars for the year lately elapsed, viz. An. ] 668. 



END OP VOLUME THIRD OF THE ORIGINAL. 



Description of an histrument invented many Years ago hy Dr. Chris- 

 TOP HER IVren, for drawing the Outlines of any Object in Perspective. 

 iV 45, p. 898. (Vol I F.J 



See fig. 1, pi. 9, wherein A is a small sight with a short arm B, which may 

 be turned round about, and moved up and down the small cylinder C D, which 

 is screwed into the piece E D, at D, this piece E D moving round the centre 

 E ; by which means the sight may be removed either towards R or F. — E F is 

 a ruler fastened on to the two rulers G G, which rulers serve both to keep the 

 square frame S S S S perpendicular, and by their sliding through the square 

 holes T T, they serve to stay the sight, either farther from or nearer to the said 

 frame ; on which frame is stuck on with a little wax the paper OOOO, where- 

 on the picture is to be drawn by the pen I. This pen I is, by a small brass 

 handle V, so fixed to the ruler H H, that the point I may be kept very firm, 

 so as always to touch the paper. — HH is a ruler, that is always moved horizon- 

 tally or parallel to itself, by means of the small strings aaaabbbb, at the end of 

 which is stuck a small pin, whose head P is the sight, which is to be moved 

 up and down on the outlines of any object. — ^I'he contrivance of the strings is 

 this : The two strings aaa, bbb, are exactly of an equal length : Two ends of 

 them are fastened into a small leaden weight QQ, which is moved in a socket 

 on the backside of the frame, and serves exactly to counterpoise the ruler HH, 

 being of equal weight with it. The other two ends of them are fastened to two 

 small pins H, H, after they have been rolled about the small pulleys N, MM, 

 LL, KK; by means of which pulleys, if the pen I be taken hold of, and moved 

 up and down the paper, the strings moving very easily, the ruler will always re- 

 main in a horizontal position. 



The manner of using it is this : Set the instrument on a table, and fix the 

 sight A at any height above the table, and at any distance from the frame SSSS 



