MAGNET1CAL OBSERVATIONS IN IRELAND. 
121 
*0001 7? has been that adopted. The standard temperature ( t ), 
to which all the results contained in the following pages are re¬ 
duced., is 60° Fahr. 
4 . All that we know of the diurnal variations of the intensity 
of the horizontal force., is due to M. Hansteen and Professor 
Christie. These writers agree in fixing the hours of minimum 
intensity at 10| a.m. The intensity then increases, and attains 
its maximum, according to Professor Christie, at about 7 2 P * M * 
The amount of this maximum is 1*0024 in summer, the minimum 
intensity being unity; but this amount, as well as the hour of 
its occurrence, changes with the season. Of the law according 
to which the force varies between its two limiting values, we 
know nothing ; and it is therefore impossible, in the present 
state of our knowledge, to apply a correction for these varia¬ 
tions. It was proposed to evade this difficulty, in the ensuing 
observations, by observing at a fixed hour. To this limitation, 
however, it was found impracticable to adhere, and the results 
still remain uncertain by the amount of the diurnal change. 
5 . The variations of the magnetic force give rise to another 
and still graver class of errors. The least experience in obser¬ 
vations of this nature is enough to prove that the horizontal in¬ 
tensity is, from some cause or other, subject to irregular fluctu¬ 
ations ; and these fluctuations, like those of the barometer in 
our climates, are much more considerable than the regular horary 
changes. It seems probable that these variations in the intensity 
of the horizontal force are, like those in its direction, not local 
phenomena, but occur at the same time at places widely sepa¬ 
rated. To eliminate them from our results, therefore, it would 
suffice to have a regular series of observations made at some fixed 
station, cotemporaneous with those made at the different sta¬ 
tions; and, if these be not very remote, we may assume that the 
variation of the observed force at each from its mean amount is 
the same as that observed at the same time at the fixed station. 
Unhappily these means of freeing the results from the admixture 
of what may be called accidental phenomena have not been 
attended to in the following, or indeed in any similar series of 
observations, and there is reason to believe that the errors due 
to this cause are the largest in amount of any by which the 
present series is affected. 
The amount of these fluctuations, from day to day, may be 
judged of from the following specimen of a series of observations 
such as that alluded to, commenced by Captain Sabine in the 
month of June 1835. The apparatus in which the needle was 
vibrated was unmoved during the continuance of the series, and 
the needle remained permanently suspended. The height of the 
