XXXVIII. An attempt to account for the regular diurnal variation of the horizontal magnetic needle; and also for its irregular variation at the time of an Aurora Borealis

The late celebrated Mr. George Graham made a great number of observations on the diurnal variation of the magnetic needle, in the years 1722 and 1723 ; but declared himself ignorant of the cause of that variation, in No 383 of the Philosophical Transactions, where many of those observations are to be found.