The accuracy- of solar observations depends primarily 

 upon the adjustment of the transit and solar attachment, the latitude 

 allowance, the skill of the operator, the accuracy of his computations 

 and atmospheric conditions; but employing proper values, which are 

 not in the least difficult of determination, the true meridian can be 

 obtained within a minute of arc or nearer, and within 10 minutes 

 of time. 



Merrimam considers the determination of the true meridian the 

 most difficult undertaking in topographical surveying. While we 

 therefore go into the theory at some length, we have never con- 

 sidered the routine either ponderous or complicated. As Professor 

 Tracy would say, there is something to remember but not much to 

 understand. Before taking up adjustments and use of the attach- 

 ment, the theory of its application and the physical concepts involved 

 should be fixed in the mind by reference to the inserted diagram. 



The Celestial Sphere, or hemisphere, is that portion of the 

 heavens which is visible to the observer at any point on the earth's 

 surface. For the sake of this argument its radius, while infinite, 

 may be assumed as reaching from the center of the earth to the sun. 



In Fig. 93 an exaggerated state of affairs is shown in which 

 So. ZPNo. is the plane of the observer's meridian and the line So. No. 

 is the true meridian lying in the observer's horizon and passing 

 through the center of the instrument. An observer stationed at the 

 instrument can see only those terrestrial objects within the range of 

 his vision, but taking the sun at 93 million miles, or a distance nearly 



160 



