504 DR. LAMONT ON THE MAGNETIC OBSERVATORY 
nation of the whole course of the phenomenon becomes impo 
sible. Astronomy belongs to the first of these classes; in i 
everything is pursued into the minutest detail, and the correct- 
ness of theory is demonstrated by such full development; it re 
quires therefore the utmost exactness in the means of observa- 
tion, so much so that it is only as the accuracy of these increases 
that we can make progress in the science. To the second o 
these classes belong all meteorological phanomena—tempera- 
ture, moisture and pressure of the atmosphere, direction and 
strength of winds, and other similar subjects. In these the pro- 
blem which presents itself differs essentially from what is the 
case in astronomy. We have not to explain the observed march 
of phzenomena, but to separate from that march all that is acci- 
dental, to draw forth that which ranges itself under laws, and to 
connect it with its efficient cause. Here we do not require the 
extreme of accuracy in final results, still less in single observa- 
tions ; but rather there is a limit of accuracy dependent on the 
amount of the effect of accidental influences, beyond which 
nothing is gained for the advancement of the object in view. 
Under which of these two classes should the investigation of 
terrestrial magnetism be ranged ? 
This question might at first have well appeared doubtful; but 
the facts which we have now before us, by our most recent re- 
searches, have brought us nearer to its decision. I will now 
indicate some of these facts, and show their application. 
After observing the march of the magnetic instruments for 
some time, it appears clearly impossible to reduce such irregu- 
lar and fluctuating movements under one Jaw, in the sense in 
which astronomical phenomena are so reduced. 
[Dr. Lamont then gives a table containing the results of a 
few days’ observations, showing their differences from an as- 
sumed mean value, and proceeds to comment on them as 
follows] :— 
We see in these observations a manifest movement dependent 
on the hour of the day; and by sufficiently long continued dura- 
tion a movement having the year for its period may be simi- 
larly traced, having deviations of the same order as those which 
take place in reference to the diurnal movement. It cannot be 
doubted that these regular periodical movements have their cause 
in some influence of the sun; but whether the deviations are 
also to be ascribed to a similar cause (i. e. to irregular solar 
