124 LIEUT.-COLONEL SABINE ON PERIODICAL LAWS DISCOVERABLE 
been made hourly from its commencement, the personal establishment left by Sir 
James Clark Ross having- been calculated with that view*. 
Having lately examined the hourly simultaneous observations of the Declination 
at Toronto and Hobarton for the years 1843, 1844 and 1845, in the course of their 
preparation for the press, I have had great satisfaction in finding that they confirm 
in a remarkable degree the anticipations which I had formed. The general evidence 
of periodicity, connected with the seasons of the year and the hours of the day in the 
mean efifects at these two distant stations, of causes which yet operate for the most 
part simultaneously at both, thus furnished by a series of hourly observations con- 
tinued for three years, is far too systematic, and rests on a basis of too long duration 
to make it probable that it will be otherwise than confirmed by the continuation of 
the series in the subsequent years ; although the exact periods, and the mean numerical 
values of the effects produced, or their proportions to each other in the different 
seasons and at the different hours may, and doubtless will, receive modifications. 
The terra ‘‘irregular” can therefore no longer be considered as correctly applied to 
this remarkable branch of the magnetic phenomena, which studied in their effects must 
now be regarded as included in the class of “periodical variations.” However (ap- 
parently) irregular may be the times of their occurrence, as general phenomena 
affecting contemporaneously parts of the globe most distant from each other, their 
effects at those stations are found to be subject to periodical laws connected with 
local seasons and local time, indicating a relation directly or indirectly to the sun’s 
place in the ecliptic, and to the earth’s diurnal rotation on its axis, and producing a 
sensible mean effect on the magnetic direction in conformity with their own pecu- 
liar laws. 
The practical bearing of this conclusion on investigations of a most interesting 
and valuable character which have recently been brought before the Royal Society, 
regarding the peculiarities of the diurnal variation in different parts of the globe and 
their physical explanation, will be evident, when it is considered that we have thus 
in each day or period of twenty-four hours two periodical variations, with laws, as 
Avill be seen in the sequel, extremely dissimilar. What commonly has received the 
name of the “ diurnal variation” of the magnetic elements, is their variation at dif- 
ferent hours of the twenty-four from a mean value obtained by summing the hourly 
observations and dividing the sum by their number; or, otherwise expressed, and with 
reference to the magnetic direction only, it is the figure described by either extremity 
of a free needle in the course of a day, under the influence of a// ‘Causes that may 
* The change from two-hourly to hourly observations was accomplished at the Ordnance observatories by 
adding, with permission of the Master- General of the Ordnance, one non-commissoned officer to the previous 
strength of the establishment. The extra pay of this artillery soldier for the particular service on which he 
was employed was fifteen pence a day, or £22 16s. id. a year. This is mentioned because it is not generally 
known at how small an expense, services of this nature within the competency of the military force stationed 
in the colonies, can be rendered, whilst the men who are so employed are available for military duty at any 
moment that may be required. 
