272 Mr. Riddle's Description ^Barclay's HydrostaticQuadiant, 



tions, and the arcs AD and EB can be read off from a gra- 

 duated circle applied to the tube, So far as the mere fixing 

 of the fluid is concerned, it is evidently of no importance in 

 what part of the tube the stop-cock is placed : as it is placed 

 in fig. 1. the two parts of the fluid will evidently be kept in 

 their situations, by the intervening column of air, in any posi- 

 tion in which, for the sake of reading, it can be required to 

 place the instrument. — So much for the principles of the instru- 

 ment. 



The tube in the instrument that has been constructed, and 

 which it is my purpose now to describe, is formed by grinding 

 two equal circular rings of about three-tenths of an inch broad 

 and seven inches interior diameter, to a sufficient depth in 

 two circular disks of plane glass, which being firmly joined 

 together, and made perfectly air-tight, the two cavities form 

 the tube ; and a stop-cock being inserted in it, the whole is 

 fastened into a brass frame, the central part of both disks of 

 glass having been previously cut out. Two brass circles 

 firmly connected together revolve round an axis passing 

 through the centre, one on the one side of the tube, and the 

 other on the other. To one of these the telescope is fixed, 

 and the circles with the telescope are moved round by a rack 

 and pinion, the latter being attached to the frame ; so that by 

 turning the nut of the pinion the telescope can be elevated to 

 any position. The only use of this part of the apparatus, 

 however, is to elevate the telescope to nearly the required po- 

 sition. The circle on the other side of the tube is the gra- 

 duated circle, the zeros standing at the points corresponding 

 to the axis of the telescope, and there are two microscopes 

 with a wire in each corresponding with the zeros of two opposite 

 vernier scales, carried round by a pinion which works in the 

 toothed edge of the same circle. The wires beins; made tan- 

 gents to the top of the fluid at the point at which the tangent 

 to the tube is a vertical line, or when the curve formed by 

 the capillary attraction is a symmetrical one, half the sum of 

 the readings is the required angle. 



But no description of an instrument can convey so good an 

 idea of its appearance and peculiarities, as may be obtained 

 from a glance at a figure of it ; and I doubt much whether a 

 person who has read only the above account of this instrument 

 will be much the wiser for it. I shall endeavour to remedy 

 this by giving a sketch and short account of each face of the 

 instrument; fig. 2. Plate III. representing what I shall call its 

 observing face, and fig. 3. Plate III. its reading face. 



AB (fig. 2.) is the telescope clamped to the toothed wheel 

 y, y, at O, O', which wheel is moved round the axis G, by the 



pinion 



