3 27 
Mr . Barlow’s observations and experiments , 
properly disposed to mask, at least in part, the terrestrial 
influence ; a method which has been long practised by mine- 
ralogists and others, when the object has been to detect 
minute attractions, I expected by this means that the cause, 
whatever it might be, that produces the daily variation, would 
exhibit itself in an encreased degree, and thereby render the 
results more perspicuous, and fix with more precision than 
has hitherto been done, the time of change and moment of 
maximum effect. 
Suppose, for example, that a finely suspended horizontal 
needle, under the natural influence of the earth, makes one 
vibration in 2", and that by masking the terrestrial influence 
by magnets properly adjusted, that time of vibration is en- 
creased to 8 " ; then it would follow that the directive power 
was reduced to one sixteenth of the former, and consequently, 
that any lateral magnetic, force acting upon the needle would 
produce an effect sixteen times greater than before ; so that if 
the former were 12', the new effect or deviation might be 
expected to amount to between three and four degrees, and 
therefore be such as to admit of distinct and satisfactory 
observation. 
A course of experiments carried on for a few days, con- 
vinced me that my ideas were correct, and that we might, 
while the needle was kept in its natural meridian, or rather 
adjusted to that direction, produce a daily variation to almost 
any amount. I obtained, for instance, the first day, a maxi- 
mum deviation of 3 0 40' ; the second, I encreased it by bring- 
ing up my magnets to 7 0 ; the third day I reduced it to 2 0 , and 
so on. I found, also, that a very considerable daily change 
would exhibit itself with the north end held to the south, to 
