OBSERVATORY. 



up for the purposes of observation by al- 

 most any common workman. 



V. Astronomical or Equatorial Sector. 

 This is an instrument for finding the dif- 

 ference' in right ascension and declina- 

 tion between two objects, the distance of 

 which is too great to be observed by the 

 micrometer. Let A B (Plate Observato- 

 ry, fig. 1.) represent an arch of a circle 

 containing ten or twelve degrees well di- 

 vided, having a strong plate, CD, for its 

 Tadius, fixed to the middle of the arch at 

 D. Let this radius be applied to the side 

 of an axis, H F I, and be moveable about 

 a joint fixed to it at F, so that the plane 

 of the sector may be always parallel to 

 the axis H I, which being parallel to the 

 axis of the earth, the plane of the sector 

 will always be parallel to the plane of 

 some hour circle. Let a telescope, C E, 

 be moveable about the centre, C, of the 

 arch, A B, from one end of it to the 

 other, by turning a screw at G, and let 

 the line of sight be parallel to the plane 

 of the sector. Now, by turning the 

 whole instrument about the axis, H I, till 

 the plane of it be successively directed, 

 first to one of the stars, and then to an- 

 other, it is easy to move the sector about 

 the joint F, into such a position, that the 

 arch, A B, when fixed, shall take in both 

 ttie stars in their passage, by the plane of 

 it, provided the difference of their de- 

 clinations does not exceed the arch, A B. 

 Then, having fixed the plane of the sec- 

 tor a little to the westward of both the 

 stars, move the telescope, C E, by the 

 screw, G, and observe by a clock the 

 time of each transit over the cross hairs, 

 and also the degrees and minutes upon 

 the arch, A B, cut by the index at each 

 transit; then in the difference of the 

 arches, the difference of the declina- 

 tions, and by the difference of the times, 

 \ye have the'difference of the right ascen- 

 sions of the stars. The dimensions of 

 this instrument are these ; the length of 

 the telescope, or the radius of the sec- 

 tor, is two feet and a half: the breadth 

 of the radius, near the end C, is an inch 

 and a half, and at the end, D, two inches : 

 the breadth of the limb, A B, is one inch 

 and a hal and its length six inches, con- 

 taining ten degrees, divided into quar- 

 ters, and numbered from either end to 

 the other. 



The telescope carries a nonius, or sub- 

 dividing plate, whose length being equal 

 to sixteen quarters of a degree, is divided 

 into fifteen equal parts, which, in effect, 

 rlivides the limb .into minutes, and, by es- 



timation, into smaller, parts The length 

 of the square axis, H I F, is eighteen 

 inches, and of the part, H I, twelve 

 inches ; and its thickness is about a quar- 

 ter of an inch. The diameters of the cir- 

 cles are each five inches ; the thickness 

 of the plates, and the other measures, 

 may be taken at the direction of a work- 

 man. This instrument may be rectified 

 for making observations in this manner : 

 By placing the intersection of the cross 

 hairs at the same distance from the plane 

 of the sector as the centre of the object- 

 glass, the plane described by the line of 

 sight, during the circular motion of the 

 telescope upon the limb, will be sum*, 

 ciently true, or free from conical curvity, 

 which may be examined by suspending a 

 long plumb-line at a convenient distance 

 from the instrument, and by fixing the 

 plane of the sector in a vertical position ; 

 and then by observing, while the tele- 

 scope is moved by the screw along the 

 limb, whether the cross-hairs appear to 

 move along the plumb-line. The axis, 

 hfo y may be elevated nearly parallel to 

 the axis of the earth, by means of a small 

 common quadrant, and its error may be 

 corrected by making the line of sight fol- 

 low the circular motion of any of the cir- 

 cumpolar stars, while the whole instru- 

 ment is moved about its axis, hfd, the 

 telescope being fixed to the limb ; for 

 this purpose, let the telescope, k I a, be 

 directed to the star , when it passes 

 over the highest point of its diurnal cir- 

 cle, and kt the division cut by the nonius 

 be noted ; then, after twelve hours, when 

 the star comes to the lowest point of its 

 circle, having turned the instrument half 

 round its axis, to bring the telescope into 

 the position m ?z, if the cross hairs cover 

 the same star supposed at 6, the eleva- 

 tion of the axis, A/o, is exactly right; 

 but if it be necessary to move the tele- 

 scope into the position, ufc, in order to 

 point to this star at c, the arch m u, which 

 measures the angle mfu, orbfc, will be 

 known ; and then the axis, A/o, must be 

 depressed half the quantity of this given 

 angle if the star passed "below 6, or 

 must be raised so much higher if above 

 it ; and then the trial must be repeated 

 till the true elevation of the axis be ob- 

 tained. 



By making the like observations upon 

 the same star on each side the pole in 

 the six o'clock hour circle, the error of 

 the axis, toward the east or west, may al- 

 so be found and corrected, till the cross 

 hairs follow the star quite round the 



