Force of the Earth's Magnetism. 21 



1821, October, Regent's Park. Th. 55°, Arcs 20° to 2°. Rate 



540".0 + 0".8+ 0".9 = 541".7. 

 1823, April, Chiswick. Th. 42°, Arcs 30° to 10°. Rate 532".4 + 2".9 



+ 3".5 = 538".8. 

 1828, March, Regent's Park. Th. 40°, Arcs 30° to 5°. Rate 534".l + 



3''.2 + 2".3 = 539".6. 

 1828, September, Chiswick. Observed by Mr. David Douglas. Th. 64°.5, 

 Arcs 30° to 5°. Rate 540".0 — 0".7 + 2".3 ■ 541".6. 

 1821, October, Regent's Park . . 541".7 



1823, April, Chiswick .... 538".8 

 1828, March, Regent's Park . . 539".6 



1828, September, Chiswick . . . 541 ".6 



Mean . 540".4 



In the case of this needle also, we may regard the mean 

 540".4 as the rate of vibration corresponding to all periods of 

 the years 1822 and 1823. 



With needle No. 5 we have, in 

 1821, August, Regent's Park, Th. 60°, Arcs 20° to 5°. Rate 726".l + 



+ 1".46 = 727".7. 

 1821, October, Regent's Park. Th. 55°, Arcs 20° to 5°. Rate 729".53 



+ 1".46 + 1".83 = 732".8. 

 1823, April, Chiswick. Th. 42°, Arcs 30° to 5°. Rate 743".7 + 5".36 



+ 3".23 = 752". 3. 

 1828, March, Regent's Park, Th. 48°, Arcs 30° to 5°. Rate 765".0 



+ 3". 6 7 + 3".32 = 772''.0. 



1828, September, Chiswick. Observed by Mr. David Douglas. Th. 66°, 

 Arcs 30° to 5°. Rate 769".4 — 1".84 + 3".34 = 770".9. 



1829, July, Chiswick. Th. 68°, Arcs 30° to 5°. Rate 772".0 — 2".47 + 

 3".35 = 772".9. 



1821, August, Regent's Park . . 727".7 



1821, October, Regent's Park . . . 732".8 



1823, April, Chiswick . . . 752".3 



1828, March, Regent's Park . . . 772".0 



1828, September, Chiswick . . . 770".9 



1829, July, Chiswick .... 772".9 



Hence we perceive that this needle gradually lost a portion 

 of its magnetism in the years 1821, 1822, and 1823, and did 

 not become stationary until some time between 1823 and 1828, 

 and that the loss was more rapid in 1821 than in 1822 and 

 1823. If, then, we distribute the total increase in the time of 

 vibration (19".5 in the eighteen months comprised between 

 October 1821, and April 1823) into a monthly increase of 

 1".5 in each of the first three months of that period, and of V 



