20 ROCHESTER ACADEMY OF SCIENCE. [ Nov. 11, 
than one or two nights in succession, no matter how brilliant the display 
may have been. From different stations they are usually reported for 
about four days in connection with a single well defined outbreak upon 
the sun. Longer continuance indicates a succession of solar outbreaks 
or the existence of a very extended area of disturbance. But even 
when disturbed areas are numerous, two, three or even more areas 
being visible, and these of large extent, it sometimes happens that no 
aurora whatever is reported and the magnets remain quiet for days 
together, tracing nearly straight lines without break or jog on the 
sensitized paper. This enigmatical behavior would be well nigh inex- 
plicable or might perhaps be referred to some unknown peculiarity of the 
explosive forces at work, were it not for the fact of periodicity, corres- 
ponding to the time of the rotation of the sun. Numerous instances 
have been noted in which auroras of very exceptional brilliancy and 
extent have recurred many times in succession at intervals of precisely 
twenty-seven days. .A very simple system of tabulation shows that this 
periodicity is a distinguishing characteristic of auroras in general. It is 
positive proof that the revolution of the sun on his axis is in some way 
involved. This being the case it is necessary to take into the account 
both solar rotation and disturbed areas, and to inquire at what point in 
their transit across the sun’s surface these areas acquire the power of 
originating auroras. It is a noteworthy fact in this connection that 
auroras and magnetic storms areas a rule characterized by great abrupt- 
ness of beginning. The excursions of the magnetic needle become 
very large almost instantaneously and require several days to die out 
gradually. In like manner auroral displays of the first magnitude burst 
forth without premonition, reaching their height at once and decreasing 
gradually, being less brilliant and reported from fewer stations on sub- 
sequent days. This behavior shows very clearly that it is not when 
solar disturbances are approaching the meridian that they produce 
auroras, in which case there would certainly be a gradual increase 
instead of such abruptness of beginning. Nor is this abruptness due, 
as might be supposed, to sudden variations in the explosive forces. It 
is in connection with precisely these grander outbreaks that there is the 
most exact periodicity and regularity of recurrence, a fact wholly in- 
consistent with the supposition that they are due to eruptive forces 
alone, such forces being necessarily irregular in their action. It is 
inconceivable that a magnetic storm, for example, due to these forces 
alone, should begin abruptly at the precise interval of six hundred and 
forty eight hours fourteen times, and should vary from this interval less 
than a few hours in several additional instances, all within the space of 
