amount of the dip of the magnetic needle , in London, &c. 21 
Gay Lussac have shown, that if any hourly variation of the 
force does obtain, it is not sufficient to produce a perceptible 
effect in a time of vibration amounting to 1 234, seconds, re- 
peated at different hours of the day and night. 
In conclusion, there appears reason to presume from the 
preceding experiments, that the dip itself may be determined 
by Meyer’s needle within a much smaller limit of uncer- 
tainty, than has hitherto been the case by needles of the 
usual construction ; as the results are subject only to those 
errors, which are reducible by repetition ; for in the ten ex- 
periments which have been submitted to the Society, the 
greatest difference of any one from the mean does not exceed 
three minutes ; the direct observation may, therefore, be 
considered as capable of sufficient precision, to justify infe- 
rences from repetition, at intervals of short duration ; both of 
the amount, and also of the uniformity, of the changes to 
which the dip is subject. 
