60 



If now, to the declination for 5fc A.M., we add the hourly rate of change succes- 

 sively, we shall have the declination for the several hours of the day, observing that 

 the first increment is for only half an hour, thus : 



Form of Daily Declination Table. 



APRIL 25, 1885. 



Hourly difference Dec. + 49" 



Dec. 5J A. M. N, 



6 



7 



8 



9 

 10 

 11 

 M. 



Hourly difference + 49" 

 . . Dec. = 13 26' 11" 

 " =13 27 00 

 " =13 27 49 

 " = 13 28 38 

 " =13 29 27 

 . . " =13 30 16 

 " =13 31 05 



The above table must be corrected for the effects of refraction, before it is set off 

 on the vertical arc of the transit. Refraction increases the apparent altitude of an 



object, and thereby affects the declination of the ob J ect dimhUshing 1 the declina - 



tion when of the 



name with the latitude - 



From the -f- sign of the " difference" of declination, we see that the declination is 

 of the same name as the latitude, whence the correction is an increment, and accordingly 

 the + sign as suffixed. This sign belongs to the refraction. 



When the object is in the meridian, refraction affects declination by its full 

 amount; but, if both the observer and the object were in the plane of the equator, 

 refraction would have no effect on the object with regard to refraction ; whence, be- 

 tween these limits, only a part of refraction is effective in changing the declination. 



Just what portion is effective, is shown by table II. of this paper. 



Thus, in the given Lat. 44, and for, say 4 hours from noon, the percentage of re- 

 fraction to be applied is .74 of that corresponding to the altitude of the object at the 

 time of observation. The sign ^ to be used must be determined, as above, by con- 

 sidering whether the sun is going north or south at the time. 



This part of the reduction of declination cannot, of course, be made till the alti- 

 tude is found at the time of observation. 



To Find the Latitude. 



Having prepared the declination for the day, as above, level up the transit care- 

 fully. Level the main telescope, observing that the vertical arc reads zero, and set 

 the polar axis to a vertical position by means of the solar telescope level. 



These points being attained, set the main telescope, pointing south. Then for a 



smith } declination ' elevate [ the 8outh end of the telesc P e > tm the vertical arc in- 

 dicates the declination thus found. 



Then, having turned the solar telescope into a vertical plane parallel with that 

 containing the optical axis of the main telescope, level it carefully and clamp it. 



A few minutes before the time of the sun's culmination, bring the telescope into 

 the vertical plane passing through the observer and the sun, and "find the sun" with 

 the solar telescope. This is readily done by varying the altitude when the sun's 

 image will appear on the diagonal eye-piece. 



Having "found the sun," bisect his image with the vertical wire, by varying the 

 azimuth with the tangent screw of the transit plate, or with that of the outer center; 

 and, simultaneously, follow him in altitude the horizontal wire bisecting the image 



till it ceases to rise, then clamp and read the vertical arc. This reading should 

 be the sum of the co-latitude and refraction, the refraction being that due to the meridian 

 altitude of the sun, which is the algebraic sum of declination ;ml co-latitude. FP.IU 

 this reading the latitude is readily deduced. With the latitude and declination 

 known, we are prepared 



