VOL. XXXVII.] PHILOSOPHICAL TRANSACTIONS. 4QI 



the instrument may occasion the images of the objects also to vibrate across one 

 another; their apparent relative motion will be very nearly in lines parallel to 

 of; and it will not be difficult to distinguish whether they coincide in crossing 

 one another, or pass at a distance : and if the objects are near one another, 

 and the telescope magnify but about 4 or 5 times, it may be held in the hand 

 without any standing support. In this manner the altitude of the sun, moon, 

 or some of the brighter stars, from the visible horizon, may be taken at sea, 

 when it is not too rough. 



Fig. 4 shows an instrument designed for this purpose; differing from the 

 foregoing description chiefly in the placing the specula and telescope, with 

 regard to the sector and index ; it has also a third speculum no, disposed ac- 

 cording to the directions when the angle is greater than QOP, whose use is to 

 observe the sun's altitude by means of the opposite part of the horizon. la 

 placing these two smaller specula, it will be further necessary to take care that 

 the speculum ikgh do not stand so as to intercept any of the rays coming from 

 the greater one fixed on the index to the third no, nor either of them hinder 

 the index from coming home to the end of the divided arch, wq is a director 

 for the sight; which is necessary when the telescope is not used. This con- 

 sists of a long narrow piece, which slides on another fixed on the back of the 

 octant, and carries at each end a sight erected perpendicularly on it : it may be 

 removed at pleasure, and exchanged for the telescope, which slides on in the 

 same manner, both serving indifferently with either of the two smaller specula. 

 The eye is to be placed close behind the sight at w; and the thread stretched 

 across the opening of the other sight at a, perpendicular to the instrument, is 

 to assist the observer in holding it in a vertical position, who is to keep this 

 thread as near as he can parallel to the horizon, and the object near the upright 

 one. How far an instrument of this kind may be of use at sea, to take the 

 distance of the moon's limb from the sun or a star, in order to find the ship's 

 longitude, when the theory of that planet is perfected, is left to trials to 

 determine. 



^n Account of the Stylus of the Ancients, and their different Sorts of Paper. 

 By the Hon. Sir John Clerk, F. R. S. Extracted by Roger Gale, Esq. F. P. 

 R.S. N°420, p. 137. 



Sir John Clerk takes occasion from some antique brass implements, found 

 near the wall of Antoninus Pius, now named Graham's Dyke, in Scotland, to 

 give this curious dissertation on the stylus, an instrument used by the ancients 

 for writing, with the figures of some of them annexed in a copper plate; two 

 of which are represented in the shape and form of the Roman fibula ; but the 

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