138 LIEUT.-COLONEL SABINE ON PERIODICAL LAWS DISCOVERABLE 
Considering Toronto in the first instance, it is obvious from this Table that the 
westerly retrogression, which is found to take place from 10 p.m. to 2 a.m. in the diurnal 
march of the magnetic declination when the whole of the observations are retained, 
is certainly chiefly, and probably entirely, occasioned by a return of the magnet from 
the disturbed position to which it had been carried by the great excess of easterly 
disturbances between the hours of 7 and 10 p.m. ; which easterly excess rapidly dimi- 
nishes after 10 p.m. When the disturbed observations are omitted, the movement of 
the north end of the needle towards the east after 6 p.m. is seen to be slower, and 
never reaches so great an easterly extreme as takes place when the disturbed obser- 
vations (which it must be remembered are only of occasional occurrence) are retained 
and allowed to influence the daily mean. By the separation of the disturbances of 
principal magnitude in the manner described in this paper, the westerly retrogression 
at the hours referred to is in fact almost eliminated, and we can scarcely doubt that 
it would be entirely so, if we possessed the means of more completely separating the 
whole of the disturbances which belong to the same class and cause as the larger 
ones ; and that by such separation the character of the residual diurnal variation 
would be entirely changed (as it is now almost entirely changed) from a double pro- 
gression having two maxima and two minima, to a single progression with one 
easterly extreme at 8 a.m., and one westerly extreme at 1 p.m. The change in the 
character of the diurnal variation at Hobarton, caused by the omission of the dis- 
turbed observations, is similar in kind to that at Toronto, though not quite so 
strongly marked. 
The change of character which thus appears to be indicated (at both stations, but 
particularly at Toronto) is a very important one, as will be readily admitted by those 
who are engaged in searching out the physical causes of these phenomena. The 
nocturnal episode, as it has been termed by Mr. Faraday, or the retrogression of the 
declination during the hours of the night, would be done away as a part of the 
regular diurnal variation ; and the physical explanation which we should have to 
seek would be that of a different phenomenon, namely, of an increased movement, 
not in the retrograde direction, but in the same direction as that of the regular 
diurnal variation, occurring at earlier hours of the night than those of the retro- 
gression ; being moreover of occasional and apparently fitful (as opposed to regular) 
occurrence, — although when the average is taken of a sufficient number of days it is 
found to be strictly periodical in its mean effects, and influential therefore on the 
mean diurnal variation of the magnet. I wish however to guard myself from being 
understood to oppose, by these remarks, the supposition that has been made of the 
existence of physical causes acting during the night antagonistic to those of the day : 
there may still be room for a supposition of this nature ; for admitting the apparent 
retrogression to be greatly lessened if not entirely eliminated by the withdrawal of 
the influence of disturbances of occasional occurrence, still it continues to be true 
that the progression of the residual diurnal variation which commences at an early 
hour of the afternoon is checked during the hours of the night, even if it be not at 
any moment actually reversed. 
