Location of the Isotonic Lines. 



Im- 



possible cause of this sudden change) to ask whether the eclipse of the 

 sun in 1806 may not have been the occasion of this phenomenon. 



The coincidence of sun spot periods with disturbances in terrestrial 

 magnetism was not then suspected, for the observations of the sun by 

 Schwabe were not commenced until 1826, and the demonstration of 

 the sun's immediate influence did not come until Carrington's happy 

 observations of the 11th of May, I860,, when the sudden out-bursts of 

 solar energy were discovered to be immediately followed by magnetic 

 disturbances at the surface of the earth. Yet before the commence- 

 ment of Schwabe's observations the keen intellect of one of the most 

 talented members of the Albany Institute had been led to suspect some 

 such instantaneous transmission of the solar force, and had suggested 

 the line of research which has brought us the proof of the origin of 

 this force. 



The genius of Mr. De Witt is admirably exhibited in this fortunate 

 suggestion, and it is highly probable that his surmise may have led to 

 the study and collection of some of the data on which the proof of the 

 accuracy of the solar spot theory is built. 



In pursuing this investigation I have taken occasion to examine the 

 present magnetic bearing of two of the lines, to which Mr. De Witt 

 refers in his paper, and found (November 29, 1884) the bearing of one 

 section of the Schenectady turnpike to be N. 30° 11', west, which is 

 4° 24' less than the value observed by Mr. De Witt, while the bearing 

 of the Western turnpike was found to be N. 56* 34 west, showing a 

 similar change of 4° 26' variation of the needle to the westward since 

 1807. 



This would show an annual movement westward of the magnetic 

 needle of 3 T % minutes at Albany since 1807. The present declination 

 of the needle at the locality in this city last mentioned is 10° 09'. 1 West 

 from the true meridian, as determined by my observation on 27th 



,ry, 1885. 

 j value of Mr. 

 magnetic needle in 



bv the ( 



affords of the moment when the needle aetually commenced its change 

 of direction of variation in the year 1806; a movement which is believed 

 to have a period of centuries even* Had it not been observed we 

 might have been left in much doubt as to the time when this import- 

 ant local change commenced. 



In the western part of northern New York the time when the needle 

 was on the true meridian has been reco rded by old surveyors as 1797. 

 ~ * In this locality this peri^dTis^robably long, possibly extending over two or 



