444 
Proceedings of Societies . [June, 
the tube from the air-spark terminal of free electricity, of the 
same name as the electricity at that terminal. The authors are 
further led to the conclusion that the discharges at the two -ter- 
minals of a tube are in the main independent, and that they are 
each determined primarily by the conditions at their own ter- 
minal, and only in a secondary degree by those at the opposite 
terminal. Having traced the relation between the two parts of 
the discharge, and having found means for controlling their 
range and influence, the authors were led to enquire whether 
there be any experimental evidence of the state of the tube 
during the occurrence of the discharge. The phenomena appear 
to require for their interpretation that, in front of the pulse 
coming from the (positive) air-spark terminal, there is, during 
the interval between the pulses, a rising negative potential : this 
is entirely swept out by the pulse as it advances along the tube ; 
after which the process is repeated. The condition of things 
behind the pulse is more difficult to determine ; but an experi- 
ment with the telephone gives reason to think that parts of the 
tube nearer to the non-air-spark end are in a condition to demand 
relief, before those nearer to the air-spark terminal have ceased 
to require it. And on this account the discharge may, perhaps, 
be more nearly represented by a lazy tongs than by a bullet. 
The marked similarities in the phenomena, and the predisposing 
circumstances of striation or non-striation, as well as in the ter- 
minal peculiarities of the two kinds of discharge, point strongly 
to the conclusions that all vacuum discharges are disruptive ; 
and that sensitive differ from non-sensitive discharges mainly in 
the scale of the discontinuity due to the disruptiveness, causing 
a difference between the two classes of phenomena analogous to 
that between impulsive and continuous forces in dynamics. 
“ On the Relation between the Diurnal Range of Magnetic 
Declination and Horizontal Force, as observed at the Royal 
Observatory, Greenwich, during the years 1841 to 1877, and the 
Period of Solar Spot Frequency,” by William Ellis, F.R.A.S. 
In this paper the author draws attention to the long series of 
magnetical observations which have been made at the Royal 
Observatory, under the direction of Sir George B. Airy, K.C.B., 
Astronomer Royal. Commencing in the year 1841, the observa- 
tions for a few years consisted of eye-readings of the various 
instruments, made every two hours ; since the year 1848 the 
motions of the magnets have been registered by photography, 
according to a plan arranged by Mr. Charles Brooke. Attention 
is briefly drawn to the question of magnetic variations, and to 
the circumstance that examination of the Greenwich records 
shows that, in addition to the ordinary diurnal and annual 
changes, there appears to exist, in the magnetic diurnal ranges, 
an inequality of marked character and of longer period, resem- 
bling in its features the well-established eleven-year sun-spot 
period. The following are the general conclusions supposed to 
be derived from the whole inquiry : — 
