160 



somewhat high one of April 1851, are the only results which 

 require notice as deviating sensibly from the means. 



To make this result perfectly accurate it should be corrected 

 for any periodical deviations from the mean of the year, to which 

 the particular months or particular hours when the observations 

 were made might be subject. But for this purpose the data are 

 not yet sufficiently ascertained, and the effect of the correction 

 would be in this series minute, because most of the observations 

 are included between May and August, when the dip is at a 

 maximum.* 



On the Isoclinal Lines in Yorkshire. 



Having ascertained the dip, and the annual change of dip, 

 of the magnetic needle at York, we may proceed to trace the 

 phaenomena of dip through other parts of the county. For this 

 purpose, observations were made in 1837, 1838, 1839, 1850, 

 1851, at many points in Yorkshire, and compared with nearly 

 contemporaneous measures at York. 



* By the Greenwich ohservatlons in 1848, 1849, 1850, the several months, com- 

 pared with the average of a year, stand thus : 



Above. 

 January, belo-w the average 



February, above 1*48 



March, above 160 



April, above 1'50 



May, above 3'35 



June, above 3"58 



July, above 072 



August, above 2-81 



September, below 

 October, below . , 

 November, below 

 December, below 



It thus appears probable that the magnetic dip has in each year a maximum about 

 the months of May and June, and a minimum in the Autumn. 



