MAGNETIC DECLINATION, AT DIFFERENT PERIODS OF THE DAY. 
641 
very far from’ inconsiderable in amount, systematic in general aspect, and apparently 
well deserving the attention of those who are occupied in the delightful and highly 
intellectual pursuit of tracing the agencies of natui’e. 
2. We perceive in the variations of position of the several months in the annual 
range, the necessity of paying regard to the period of the year, as well as to the 
period of the day, at which observations have been made, which do not include long 
intervals, and from which nevertheless inferences are drawn in respect to secular 
change. Such observations, when not those of a fixed observatory, are usually made 
at some hour in the day-time, when it needs only a glance at the Plate to perceive that 
Annual as well as Diurnal variation-corrections are required, unless the month as 
well as the hour is the same in the earlier and later observations. A table of correc- 
tions for every hour of the day to the mean value in each month — corrections derived, 
as in the instances now before the Society, from a series of strictly comparable obser- 
vations continued for several years — should be considered, not merely as a desirable, 
but as an almost indispensable provision, in countries where magnetic surveys are 
conducted with the degree of perfection of which they are now susceptible. 
Woolwich^ April 29, 18.51 ; revised October 1851. 
