OBSERVATORY. 



tcuty tevel. The Hanging level must then 

 be fixed to the brass rod by two hooks of 

 equal length, and made truly parallel to 

 it : for this purpose make the polar axis 

 perpendicular or nearly perpendicular to 

 the horizon ; then adjust the level by the 

 pinion of the declination semicircle ; re- 

 verse the level, and if it be wrong 1 , cor- 

 rect half the error by a small steel screw 

 that lies under one end of the level, and 

 the other half-error by the pinion of the 

 declination semi-circle ; repeat this till 

 the bubble be right in both positions. In 

 order to make the brass rod on which the 

 level is suspended at right angles to the 

 axis of motion of the telescope or line of 

 collimation, make the polar axis horizon- 

 tal, or nearly so ; set the declination semi- 

 circle to 0, turn the hour circle till the 

 bubble comes right ; then turn the de- 

 clination circle to 90 ; adjust the bubble 

 by raising or depressing the polar axis 

 (first by hand till it be nearly right, af- 

 terwards tighten with an ivory key the 

 socket which runs on the arch with the 

 polar axis, and then apply the same ivory 

 key to the adjusting screw at the end of 

 the said arch till the bubble come quite 

 right) ; then turn the declination circle to 

 the opposite 90 ; if the level be not then 

 right, correct half the error by the afore- 

 said adjusting screw at the end of the 

 arch, and the other half error by the two 

 screws which raise or depress the end of 

 the brass rod. The polar axis remaining 

 yearly horizontal as before, and the de- 

 clination semi-circle at 0, adjust the bub- 

 ble by the hour-circle ; then turn the de- 

 clination semi-circle to 90, and adjust the 

 bubble by raising or depressing the polar 

 axis; then turn the hour-circle twelve 

 hours ; and if the bubble be wrong, cor- 

 rect half the error by the polar axis, and 

 the other half error by the two pair of 

 capstan screws at the feet of the two sup- 

 ports, on one side of the axis of motion of 

 the telescope ; and thus this axis will be 

 at right angles to the polar axis. The next 

 adjustment is to make the centre of cross 

 hairs remain on the same object, while you 

 turn the eye-tube quite round by the pi- 

 nion of the refraction apparatus : for this 

 adjustment, set the index on the slide to 

 the first division on the dove-tail ; and set 

 the division marked 18" on the refraction 

 circle to its index ; then look through the 

 telescope, and with the pinion turn the 

 eye-tube quite round; and if the centre 

 of the hairs does not remain on the same 

 spot during that revolution, it must be 

 corrected by the four small screws, two 

 and two at a time (which you will find 

 upon unscrewing the. nearest end of 



the eye-tube that contains the first eye- 

 glass) ; repeat this correction till the 

 centre of the hairs remain on the spot 

 you are looking at during an entire revo- 

 lution. 



In order to make the line of collimation 

 parallel to the brass rod on which the le- 

 vel hangs, set the polar axis horizontal, 

 and the declination circle to 90, adjust 

 the level by the polar axis ; look through 

 the telescope on some distant horizontal 

 object, covered by the centre of the cross 

 hairs ; then invert the telescope, which is 

 done by turning the hour-circle half 

 round ; and if the centre of the cross 

 hairs does not cover the same object as 

 before, correct half the error by the up- 

 permost and lowermost of the four small 

 screws at the eye-end of the large tube of 

 the telescope ; this correction will give a 

 second object, now covered by the centre 

 of the hairs, which must be adopted in- 

 stead of the first object: then invert the 

 telescope as before ; and if the second ob- 

 ject be not covered by the centre of the 

 nairs, correct half the error by the same 

 two screws which were used before : this 

 correction will give a third object, now 

 covered by the centre of the hairs, which 

 must be adopted instead of the second ob- 

 ject ; repeat this operation till no error 

 remains ; then set the hour-circle exactly 

 to twelve hours (the declination circle re~ 

 maining at 90 as before) ; and if the cen- 

 tre of the cross hairs does not cover the 

 last object fixed on, set it to that object by 

 the two remaining small screws at the 

 eye-end of the large tube, and then the 

 line of collimation will be parallel to the 

 brass rod. For rectifying the nonius of 

 the declination and equatorial circles', 

 lower the telescope as many degrees, 

 minutes, and seconds, below or JE on 

 the declination semi-circle, as are equal to 

 the complement of the latitude ; then ele- 

 vate the polar axis till the bubble be ho- 

 rizontal, and thus the equatorial circle 

 will be elevated to the co-latitude of the 

 place ; set this circle to six hours ; adjust 

 the level by the pinion of the declination, 

 circle; then turn the equatorial circle 

 exactly twelve hours from the last posi- 

 tion ; and if the level be not right, correct 

 one half of the error by the equatorial 

 circle, and the other half by the declina- 

 tion circle ; then turn the equatorial cir- 

 cle back again exactly twelve hours from 

 the last position ; and if the level be still 

 wrong, repeat the correction as before till 

 it be right, when turned to either position ; 

 that being done, set the nonius of the 

 equatorial circle exactly to six hours, and 

 the nonius of tf\e declination circle exactly 



