y i. ul rant. 



subdivisions, was inserted in this quadrant by Bird, 

 bi'twivn tin- two original divided arcs, and all the ob- 

 aervntii.n-, since- made with that instrument are referred 

 to this new arch. 



Descrip- 

 tion of 

 Ilini'n Mil- 



Ma- 

 dram. 



Cole's Qua- 

 drant by a 

 single lie- 

 flection. 

 PLATE 



( (i CJ.XXVI, 



Fig. 10. 



2. Description of Bird's Mural Quadrant. 



The Mural Quadrant of Mr. Bird was constructed on 

 the model of Graham's, and was made entirely of brass. 

 It was erected in the time of Dr. Bradley in 1750, and 

 was used by that astronomer and Dr. Maskelyne in their 

 most valuable observations. The radius of that instru- 

 ment is eight feet. Near the eye-piece of the telescope 

 there is a good micrometer, for giving it a slow mo- 

 tion, and for measuring the number of seconds that 

 the reading lines of the vernier want of coincidence in 

 any observation. As Mr. Smenton recommended, how- 

 ever, the vernier is no longer used, and the last 5' 

 space in the observed are is subdivided by the screw. 



Mr. Pond having had reason for suspecting the accu- 

 racy of the total arc of the quadrant, Mr. Troughton ex- 

 amined it by an apparatus contrived for the purpose. 

 He found the total length of the quadrantal arch 7" 

 too small, and he discovered a similar error of 2", 

 arising from the wear of the axis of motion. In 

 the intermediate division he could not detect more than 

 one second of inequality. The difference of the read- 

 ings of the two arcs has never been observed to exceed 

 4", which proves the great accuracy of Bird's gradua- 

 tion. The diminution of 7" in the total arch seems 

 to be occasioned solely by a change of figure in the 

 space of 48 years, as Dr. Bradley found the arc to be 

 exactly 90 in 1759- 



Mr. Troughton has not examined the quadrantal arch 

 of Graham's quadrant, but he is of opinion that, being 

 made of iron, it has preserved its form better than 

 Bird's, which was made of brass. 



8. Cole's Quadrant by a single Reflection. 



This instrument, invented by Mr. Benjamin Cole, is 

 composed of a moveable radius AB,PlateCCCCLXXVI. 

 Fig. 10. a quadrantal arch DE, three vanes A, B, C, and 

 a vernier FG. The moveable radius is a piece of wood 

 about 2 inches long and l thick. The quadrantal arch 

 is also of wood, and has a radius of 9 inches, being di- 

 vided into degrees and third parts of a degree. The sight 

 vane A, a thin piece of brass about 2 inches high and 

 1 broad, is placed perpendicularly at the end of the 

 radius A, and through the middle of it is a small 

 hole, through which the coincidence of the horizon 

 and solar spot is to be viewed. The horizon vane 

 B, about 1 inch broad, and 2^ high, has a slit cut 

 through it about 1 inch long and broad. It is fixed 

 in the centre pin of the quadrant perpendicularly, and 

 is always inclined 45 to the sight vane. The shade 

 vane C consists of two brass plates, one of which revolv- 

 ing on an arm, is about 4^ inches long, and f ths of an 

 inch broad. It is pinned at one end to the upper limb 

 of the quadrant by a screw, about which it has a small 

 motion. The other end lies in the arch, and the lower 



QUADRANT. 287 



to a degree on the upper limb of the quadrant, and Quadrant, 

 turn the screw on the back side of the limb forward or 

 backward til) the hole in the sight vane, the centre of 

 the glass, and the sunk spot in the horizon vane, all lie 

 in the same straight line. In order to take the altitude of 

 the sun by that quadrant, turn your back to the sun, 

 and hold the instrument by the radius AB with your 

 right hand, till it is in a vertical plane passing 

 through the sun, then looking through the sight vane 

 and the horizon vane till you see the horizon, move the 

 quadruntal arch upward with the left hand, till the 

 shadow of the sight vane or the solar spot formed by 

 the lens fall directly on the slit in the horizon vane. 

 When that is done, the part of the quadrant raised 

 above G or S, according as the solar spot or shade is 

 viewed, will be the altitude of the sun required. 



4. Description of Gunlers Quadrant. 



This quadrant, invented by Edmund Gunter, Profes- 

 eor of Astronomy in Gresham College, has been in use i'on of 

 since 1618. It is commonly made of wood, with its Gunwr'i 

 limb divided into 90, and two sight vanes placed in ( * uadrtnu 

 one of the radii next the division of 90. It has like- 

 wise a stereographical projection of the sphere on the 

 plane of the equinoctial, and a kalendar of the months 

 close to the divisions on the limb. The indications on 

 the limb are obtained by a plummet-line with a move- 

 able bead. The places of five stars are also laid down 

 on the quadrant, so that a series of astronomical pro- 

 blems can be performed with the quadrant in the same 

 manner as with a celestial globe. Thus, 1st, If the 

 thread is laid to the day of the month, it will cut on 

 the limb the degree of the sun's meridian altitude, and 

 vice versa. 2d, If the bead is slid on the thread to the 

 sun's place in the ecliptic, and if the thread be made 

 to cut on the limb the sun's meridian altitude, as ob- 

 served with the quadrant, the bead will fall upon the 

 hour of the day. 3d, If the bead is first laid to the sun's 

 place, and then on the given hour of the day, the 

 thread will cut the sun's altitude on the limb. 4tb, If 

 the bead is set to the sun's place, and the thread mov- 

 ed to the line of declination, the bead will cut the 

 sun's declination. 5th, The bead being rectified to the 

 hour of the day, as in art. 2, and the sun's altitude ob- 

 served, bring the thread to the complement of the alti- 

 tude, and the bead will point out the sun's azimuth 

 among the azimuth lines. 6th, The thread being laid 

 upon the sun's place in the ecliptic, it will point out 

 on the limb his right ascension, and vice versa. 7th, 

 In order to find the hour of the night from any of the 

 five stars laid down on the limb, put the bead to the 

 star to be observed, and, by art. 2, find how many 

 hours it is from the meridian. From the star's right 

 ascension subtract the sun's right ascension in time, 

 and add this difference to the observed hour of the 

 star from the meridian, the sum is the hour of the 

 night. 



5. Description qfSulfon's or Collins' Quadrant. 



edge of the arm is directed to the middle of the centre This quadrant, see Fig. 11, fitted to the latitude of Descrip- 



pin. The other plate, which is properly the vane, is London, contains a stereographic projection of one quar- tion of Sut- 



about two inches long, and is fixed at right angles to ter of the sphere between the tropics, the eye being in ton 'f * 



the first plate, at the distance of about half an inch from its north pole. The lines which run from right to left ' 



the end next the arch. This vane may be used either are parallels of altitude, and those which cross them are p", a j" 



by the shadow which it casts, or by the solar spot azimuths. The lesser of the two circles which bound 



formed by a convex lens placed within it. In order to the projection is one-fourth of the tropic of Capricorn, 



rectify this vane, set the line C of the vernier opposite and the greater is one-fourth of that of Cancer. From 



