80 



19. Remarks. When the watch is corrected by either method, it will give 

 the value of t in time directly as follows : Note the time of an observation. Apply 

 to this time the equation of time with its sign, as given in the Solar Ephemeris Table 

 of the Nautical Almanac. The result will give the apparent time of the observation 

 from apparent noon, -f- when the sun is west of the meridian, and when it is east. 

 This found is the required value of t. 



2(K Reducing Observations. 



<i. Conditions. Let the notation be as before. 



Let t = the sun's apparent hour angle at the time of the observation, that is the 

 local apparent time from apparent noon This is -}- when W. and when E. of the 

 meridian, or -f- for P.M., and for A.M. times. 



Let the value of t be found by 18 or 19, and reduced to arc at the rate of one 

 degree of arc to each four minutes of time, the work being carried out to seconds 

 of arc. 



O. Directions. For finding A when J, f, and z are given. Find A from the 

 following equations. 





Example. <* = 18 30' 20" N. z = 52 43' 30" .t = 55 46' 32'/5 

 Cos d = 9.97765541 

 Sin t= 9.91 74225 



Sin z = 9. 9007700 J 

 Sin A = 9.9943079 J 

 A = 9915'22':5 



2O Remarks. The value of A as determined in these examples is greater 

 than 90, because the sun is south of the zenith. The value of t used in the second 

 example was found from the first, hence the exact check. It may be noticed how 

 much less figuring is required in the second example than in the first. It should 

 be noted, however, that more than one check is figured out in the first example, 

 and so more than the necessary figures shown. The value of A is carried out with 

 exactness in order that the process' may be fully illustrated. 



21. Summary. Several courses are hereby opened to the surveyor. This 

 is done that he may have the more checks at his command, and so make certain of 

 his work, It may be well to indicate these courses in a catalogued form for easy 

 reference. The courses are 



The processes of 1O, 12, and 14 or 15. 



The processes of 1C, 12, and 14 or 15, and thence 



16, a, or 16, d, and 18 or 19 and 2O. 



22. Cautionary. Keep the levels and the vernier of the vertical circle in 

 good adjustment. Also keep the adjustment of the axes of the instrument, the 

 transit axis and the vertical axis, in good order. 



23. General Remarks. It will be seen that in doing solar work with these 

 attachments in the manner explained above, the observation of the sun depends on 

 the ordinary line of sight of the telescope exactly as in all Geodesic work. 



For this reason no extra adjustments are required. The accuracy of the observa- 

 tion in no way depends on these attachments, which are merely conveniences to 

 enable one to make solar observations with the ease and precision of ordinary 

 terrestrial work. 



Other Solar Attachments are mechanical devices requiring special adjustments, 

 and considerable care is necessary to keep these adjustments perfect, while they 

 cause some degree of anxiety and doubt in the mind of the engineer as to whether 

 tln-v an- ijuiti- perfect or not. 



With t his invention all these sources of anxiety are avoided, the solar observation 

 being made with the telescope of the transit itself, while it has the advantage of 

 IHMIII: applicable to every surveyors' and engineers' t ransit. is so light as not to add 

 appreciably to the weight of the instrument, so simple as to require no special pro- 

 vision for its care, and so cheap as to be within the reach of every surveyor. 



