Section III., 1914 [io5] Trans. R.S.C. 



On the Diurnal Changes in Magnetic Declination at Agincourt, 

 1902-1912. 



By W. E. W. Jackson, M.A. 



Presented by R. F. Stupart, F. R.S.C. 



(Read May 27, 1914). 



The changes which take place in the direction of the Earth's 

 magnetic field at any one station must be thoroughly known before 

 we can say definitely what direction a magnet, if suspended freely, 

 will take with reference to a meridian. Before the establishment 

 of fixed observatories it was known that there were certain times of the 

 day when the needle would point in a different direction to other 

 times, but as the angular values were small and of little moment for 

 mariner's purposes, no thorough study was made of them. In the 

 early part of the 19th century it was noted by different philosophers, 

 amongst whom was von Humboldt, that the diurnal range was not 

 always of the same amount, that it varied from place to place, and also 

 with the season of the year. 



At the request of the Royal Society and the British Association, 

 due to von Humboldts efforts, several observatories were authorized 

 by the British Government to be established in different parts of the 

 British possessions for the investigation of the changes taking place 

 in the Earth's magnetism, and one of these was established at Toronto 

 in 1839, where it was kept in operation until 1897 when, on account 

 of electric disturbance from the trolley system, it became necessary 

 to move to the present quarters at Agincourt, some 12 miles distant. 



Since 1880 photographic records of the changes in the Earth's 

 magnetic forces have been obtained, whilst prior to that time the 

 records were all from actual eye observation. 



It is proposed to present in this paper some of the results obtained 

 from an analysis of the curves secured at Agincourt during the eleven 

 year period 1902 to 1912. 



Measurements have been made from the curves at every hour 

 of the 24 for all days, and these hourly values have been combined 

 for each calendar month, and taking in succession the difference 

 of each hourly value from the arithmetic mean for the month, we 

 obtain what is known as the diurnal inequality. The smooth pro- 



