255 
the paper may require some correction ; still it is believed 
that the ranges of the two periods are not identical. 
Another period of change having a mean duration of rather 
over eighteen months is then referred to. The Author was 
first led to it from a discussion of the Greenwich magnetical 
observations for the years 1848 to 1859; and it has been 
confirmed by the results of a discussion of temperature 
observations made at Brussels, in Europe, and at Yakou'tsk, 
in Asia. It is obvious that this period will at times interfere 
sensibly with the shorter one, and it is probable that some 
of the cases which have been called exceptional may be due 
to this interference. 
As it may perhaps excite surprise that in this investiga- 
tion no use has been made of the fine series of observations 
taken at the British Colonial Observatories, it is remarked 
that a little consideration will serve to show that in the early 
stages, at least, of an inquiry like the present, little or no 
reliance could be placed on results derived from series of 
observations in which every seventh day, or fifty-two days in 
the year, were complete blanks. It has, in fact, been found 
that the omission of a single day in a year will in some cases 
produce a very sensible effect upon the final results for the 
magnetic period. Hence it is that the Author regards some 
of the values he has obtained for this period as being open to 
correction when he may have an opportunity of discussing 
more complete sets of observations. General Sabine, in his 
very elaborate discussions of the magnetical observations made 
at the colonial observatories, separates the larger movements 
of the needle from the general mass, and treats them as 
extraordinary disturbances, assuming apparently that the 
two classes of ordinary and extraordinary disturbances are 
due to different causes. The Author has, however, not 
ventured to adopt this mode of proceeding, but has preferred 
to regard all the movements of the magnet, whether large or 
small, as having a common origin, and his discovery of the 
