The Variation of the Needle. 



sextant), a simple computation then giving the direction of the true 

 meridian closely enough for purposes of navigation. The difference 

 between the magnetic and true azimut h of the sun would, of course, 

 be the declination of the needle. It is evident that such observations 

 cannot be as accurate as those taken on terra firma, but as they are 

 sufficiently correct for navigation, these methods will probably always 

 be used at sea. 



The form of azimuth compass employed on hydrographic surveys 

 from boats will give some idea of the larger instrument and is here 

 exhibited. It is quite convenient when magnetic bearings are desired 

 from a float that is rising and falling with the waves, the needle carry- 

 ing a circle divided to degrees of arc read by reflection in the line of 

 sight by means of a prism possessing magnifying power. 



Whether the azimuth compass and the frequent use of the sun in 

 connection with it. as an astronomical signal, suggested the solar com- 

 pass, cannot be told, but the ub-i-nce of a clear unobstructed horizon 

 has prevented the use of nautical methods inland, and land surveyors 

 have in former years to a great extent made the pole star their point 

 of reference. 



Tables, published in text-books on surveying, give the mean solar 

 time when that star (a Ursa Minoris) is on the true meridian; 

 and, if this occurs at some inconvenient hour of the night — or in the 

 day-time when the star is invisible — other tables in the same books 

 give the times of elongation of the star, its easting or westing from the 

 meridian and the azimuth angle between it and the true meridian at 

 various latitudes. The trouble with such tables is that in time they 

 become obsolete, as even the fixed stars are less permanent in their 

 apparent positions than most people imagine. 



Yet local surveyors often continue to use the tables long after the 

 periods for which they were designed, believing the contents of their 

 text-books to be indisputable authority, and the "fixed stars " to be 



To remedy this source of error I have taken occasion in my reports 

 on the progress of the Adirondack survey to publish new tables of the 

 time of upper transit of Polaris, and of its eastern and western elonga- 

 tions, which have been regularly computed in the survey office for each 

 year, and printed and distributed to the assistants on the survey need- 

 ing them. 



It requires patience and time however to observe the pole star either 

 on the meridian or at elongation, and surveyors usually do not attempt 

 other methods of star-work when using the small forms of engineer's 



