Location of the Isogonic Lines. 



transits. When it is considered that after waiting at one place for a 

 day or two for a clear night to observe the star, a single cloud flitting 

 across the sky will often prevent observation at the instant that it 

 should be taken, we shall not be surprised that inventive and thought- 

 ful men have made efforts to discover other methods for determining 

 the true meridian which should be sufficiently accurate, and be availa- 

 ble at different hours, and particularly during the natural time for 

 work, during the day. 



The great equatorial telescopes of observatories make the stars visi- 

 ble even when the rays of the sun fill the sky with polarized light; and 

 it must be believed that this fact and the peculiar mounting of this 

 form of instrument led to the construction of the first real solar com- 

 pass, which may be regarded as a species of portable equatorial instru- 



The solar compass was invented and patented by Mr. William A. 

 Burt, an American surveyor, in 1835. 



The horizontal limb, sights, declination and hour arcs, are es- 

 sential parts of this useful instrument, which has been so extensively 

 employed in the United States Government surveys of the public lands. 



The success of the solar compass as a practical implement in sur- 

 veying led to many attempts at its improvement, but the patents held 

 by the inventor covered most of the devices proposed and prevented 

 the combination of the improvements until the expiration of the patent 

 a few years since. Many surveyors , also, who were unfamiliar with 

 the instrument, and from old habits could not be brought to see the 

 utility of frequent determinations of the true meridian, still made an- 

 nual or even less frequent observations of Polaris, in accordance with 

 the old tables, when they desired to orient a survey: and believed that 

 the constantly changing declination of the sun, and its large size as an 

 object of observation, made it an inaccurate signal. They were not as 

 familiar as navigators, with the reliable nature of our great luminary, 

 and did not consider that in land surveys the roughness of the ground 

 traversed requires more frequent determinations of the direction of 



star. Every few miles in land surveys the direction of the lines run 

 requires testing and adjustment to the true meridian to the degree of 

 closeness that is read upon the limb of the surveyor's instrument. 



tained within certain practical limits when the data for observation 

 were carefully arranged and computed in advance for the time of obser- 

 vation; and it was not until the so-called "solar attachment" for 



