61 



To Find the Meridian. 



(a.) As for finding latitude, level up the instrument carefully, the vernier of plate 

 Clamped, reading zero. 



(b.) Point the telescope to the sun to find his altitude for the refraction. This 

 can be found with sufficient accuracy by turning the telescope, till the shadow of a 

 pencil held across the end, or till the shadow of the screws on the side, are parallel 

 with the tube. 



(c.) The refraction corresponding to this altitude must be multiplied by the cor- 

 responding coefficient, for the time from noon and the latitude, and applied to the 

 declination, as per instructions above, for the corrected declination. 



(d.) Point the telescope to the south, eiJvatSg | the 8outh end for south | dec11 " 

 nation, till the vertical arc reads the corrected declination, and clamp the vertical 

 arc. 



(e.) The main telescope being dipped to the corrected decimation, level the solar 

 telescope by means of its level, being careful to do so when it is in a vertical plane 

 parallel with that containing the optical axis of the main telescope, for only when it 

 is in this plane can the declination be properly set off. 



(f.) Elevate the south end of the main telescope to the co-latitude, by means of 

 the vertical arc, and turn the telescope approximately into the meridian, by means 

 of the magnetic needle. 



(g.) " Find the sun " with the solar telescope. This is done by turning the whole in- 

 strument in azimuth, on its outer center, simultaneously with a motion of the solar teles- 

 cope in right ascension, till the sun's image is seen in the eye-end of solar telescope. 

 Bisect the image, as nearly as may be, by the two motions above named clamp 

 and complete the bisection, by both wires, or by the wires forming a square, by means 

 of the transit's lower tangent screw, and by that of the solar telescope. If the image 

 of the sun should be so large that it cannot all be seen from one position of the eye, 

 look around it by moving the eye around it in such a manner as to see the entire cir- 

 cumference, and bring the cross-wires on the four sides of the image normal to then* 

 respective sides, by means of the motion in azimuth and the motion of the solar 

 telescope, as above described. This being attained, the optical axis of the main teles- 

 cope should be in the astronomical meridian. Refer to an azimuth mark, and repeat 

 the operation. The above is called a direct observation. 



(h.) To make a reverse observation. Having made the direct observation, turn the 

 whole instrument 180 in azimuth, and set the co-latitude off on the opposite side 

 of the vertical arc. Also turn the solar telescope 180 , and proceed as before. The 

 object of repetition is to eliminate personal non-precision and possible errors in 

 manipulation, while the object of reversing is to eliminate any possible remaining 

 errors of adjustment of the instrument. The prudent surveyor will not trust his work 

 without such verification, and he will take the mean of both observations. 



Remarks. 



(1.) To unscrew the .solar attachment from the packing-piece in the box, first 

 release the clamp and tangent screw, and then turn carefully the milled-edged disk or 

 base -plate a few turns to the left. To screw the solar attachment to the instrument, 

 turn this milled-edged disk from left to right around the screw on top of the main 

 telescope without revolving the solar attachment. To insure a perfect contact of 

 screw-shoulder against the flange, on which depends the permanency of the adjust- 

 ment of the polar axis to the main telescope, it is necessary that these parts be free 

 from dust, grit, or dirt of any kind. 



(2.) The auxiliary or latitude level, if one is ordered, attaches in the same manner 

 to the end of the cross axis on the side of the vertical circle. 



(3.) The latitude level is used to facilitate during repeated observations the reset- 

 ting of the polar axis to the co-latitude, assuming that the polar axis has been pre- 

 viously set to be at right angles to. the main telescope by its milled capstan-headed 

 screws and the solar level, the polar axis being placed in its position for an observation 

 with more facility and precision with this level than by reading the vertical arc. 



NOTE. The great utility of this auxiliary, or level attachment, is seen in the setting of grades. Two of 

 these levels being applied to the telescope of a pivot-levelling instrument one on each side or one on each 

 end of the cross-axis of a transit telescope, and one of them being adjusted to the up, the other to the down, 

 grade, the engineer may work in either direction on his grade with the same facility that he would on a level 

 line. 



