358 



ciety's garden, at Chiswick ; the distance apart is about five miles, 

 but the direction is as nearly as possible that of the line of equal 

 dip. 



The apparatus, modes of observing, and needles employed, are 

 fully described. The needles were four in number ; one, of the or- 

 dinary construction ; a second, fitted with Professor Meyer's appa- 

 ratus for avoiding the errors arising from the non-coincidence of the 

 centres of gravity and motion ; a third, having a cross of wires at- 

 tached to the axis, on the well-known plan of Dr. Mitchell ; and a 

 fourth, devised by Mr. Dollond, the middle of which is a cube per- 

 forated at right angles, so that the axis may be inserted in eight dif- 

 ferent ways. 



In addition to his own apparatus and needles, the author obtained 

 from the Colonial Department the use of a smaller apparatus, with a 

 needle on Professor Meyer's plan, the same which was used by Cap- 

 tain Franklin 1 , on his last land expedition. The observations with 

 this apparatus were made by Mr. David Douglas, of the Horticul- 

 tural Society. The results were as follows : 



With the ordinary needle 69 46'- 1 



With Meyer's needle 69 47 '4 



With a needle having an adjusted axis . . 69 38 '3 



With Mr. Dollond's needle 69 51 -7 



With the smaller apparatus 69 51 '4 



Dip in London, in August, 1828 69 47 "0 



From the observations of 1821 and 1828, the author finds a de- 

 crease in the dip in London, of 17'*5 in seven years, or an annual 

 decrease of 2'*5. 



The average annual decrease for the century preceding 1821 ap- 

 pears, from the most authentic observations, to have exceeded 3'. 

 On examining the series of observations made on the dip in Paris 

 since 1 798, by MM. Humboldt, Gay Lussac, and Arago, the author 

 finds a corresponding indication of a recent diminution in the yearly 

 decrease of the dip ; it appearing, by those observations, that the 

 average yearly decrease, in the first half of the period between 1798 

 and 1828, exceeded 4''75 ; and in the second half fell short of 3'. 

 He concludes by remarking, that a repetition of the observations in 

 London, at the expiration of another seven years, and a continuation 

 of those at Paris, will probably afford a decisive indication on this 

 point ; and notices, in case the annual change shall prove to be di- 

 minishing in this part of the world, the importance of determining 

 the precise period at which the dip shall become stationary, and the 

 minimum to which it shall then have arrived. 



