397 
and on the Dip of the magnetic JV'eedle. 
to be* 75 0 10'; this, when compared with many, and very 
accurate observations made by Mr. Cavendish, with several 
needles in the yearf 1775, who found it to be 72*30', makes 
the decrease in this period of 55 years on a mean, 2 ',9 per 
annum. And from a comparison of my own observations of 
the dip in 1805, which was 70° 21', with the above of Mr. 
Cavendish in 1775, its annual decrease, on a mean, appears 
to have been 4', 3 ; and its progressive annual decrease on a 
mean in the above mentioned period of 30 years i',4. 
I cannot conclude this Paper without expressing my regret, 
that so little avail should have been made of the numerous 
opportunities which have been afforded to travellers and 
others in the last century for making accurate observations 
with proper instruments, at land, on the variation in different 
parts of the world : such observations would probably have 
afforded some curious and useful facts which would have ma- 
terially assisted in forming a theory much more certain than 
what we at present possess ; the present received opinion of 
the cause of the diurnal alteration of variation would be con- 
firmed or invalidated ; its quantity of effect in different places, 
a most desirable acquisition, would be ascertained ; and we 
should be put in possession of more valuable and correct 
information on the variation than can be derived from obser- 
vations made with the common azimuth compass, even at 
land, owing to its imperfect construction. The variation thus 
accurately obtained at any one period, compared with the 
variation correctly ascertained at a subsequent period, would 
• Longitude and Latitude found by Dipping Needle, p. 7-— 94.. 
f Phil. Trans. Vol. LXVI. p. 400. 
