DIURNAL VARIATION OF THE MAGNETIC NEEDLE. 



The Diurnal Variation of the Magnetic Needle was a subject which attracted the at- 

 tention of scientific men about the year 1759, when many experiments were made. Its 

 motion westward was observed to commence about 8 a.m., and continue until 2 p.m., 

 when it became stationary for some time, and afterwards moving gradually back until 

 it attained its first position, its utmost extent of variation being 19" 4"'. It was also ob- 

 seived that at the time an Aurora Borealis was seen, its variation though irregular, was 

 slowly eastward in the morning and westward in the evening, and in the night suddenly 

 both ways in a very short time. These phenomena were attributed to the power of the 

 sun in heating the eastern magnetic properties of the earth in the morning, and those 

 of the western in the evening ; this was demonstrated by placing a strong magnet on 

 each side of a compass, so as to keep the needle in the magnetic meridian, and alter- 

 nately screening the sun from each magnet, it was observed, that when the sun was 

 shining on the eastern magnet only, that the needle moved to the westward. This 

 hypothesis was also corroborated by the fact, that the diurnal variation was observed 

 to be greater in the summer than in the winter at London, as will appear by the 

 following table of the mean diurnal variation for each of the twelve months of the 

 year 1759: 



The maximum being in June, and the minimum being in December. Thus the regular 

 diurnal variation was concluded to be occasioned by the heat of the sun, which, however, 



