192 



The Variation of the Needle. 



engineer's transits was devised; and its great utility, in localities where 

 l(K-;iI attraction is met with, demonstrated ; that surveyors in the eastern 

 States came to regard this instrument as a valuable convenience. No 

 sooner, however, was the utility of this new device admitted than 

 various forms of solar attachments to transits made their appearance, 

 and even a form of "pocket solar" was contrived. Almost all these 

 forms of apparatus are before you this evening and T shall take 

 pleasure, a little later, in exhibiting and explaining the use of these 

 instruments. Among them will be seen the original form of Burt's 

 solar compass, the Gurley and Holmes' solar transits, Stackpole's solar 

 attachment, Sagmuller's solar telescope, the pocket solar, etc. The 

 general principle of these instruments is nearly the same. A deter- 

 mination of the latitude of the place of observation is first made. The 

 declination of the sun (or star, if a .-tar be observed),* is then laid off 

 on a declination arc, centered over the instrument upon a polar axis, 

 and the instrument thus set with its hour circle in a plane parallel 

 to the plane of the earth's equator, the sun (or star) can only be ob- 

 served through the declination telescope, when the instrument is in the 

 meridian, so that the bringing of the instrument into the meridian is 

 then readily accomplished by a few turns of the tangent screw. 



Recent Observations in Northern New York. 

 As the result of my personal observations of the magnetic declina- 

 tion in the northern district of New York, during a long period of 

 years, I am able to place before you the first map that shows with any 

 minutiae the location of the isogonic lines, or lines of equal magnetic 

 declination in the Adirondack wilderness region and adjacent settled 

 districts. 



Some of the more recent of the magnetic observations that I have 

 made are among the most interesting, as they have enabled me to 

 ascertain what the declination of the needle of the compass surveyors 

 was at periods prior to the Revolution. Not only the error of the 

 pointing of the needle used more than one hundred years ago is thus 

 obtained, but at times valuable evidence indicating the date of the 

 change in the directive motion of the controlling magnetic force at 

 that distant period is found with accuracy. This information is an 

 element of the greatest value in the study of terrestrial magnetism, 

 and the method, although indirect, is not so obscure as to be difficult 



improved forms, the telescope < 

 any declination and hour angle 



