1879.] 
Proceedings of Societies. 
817 
the motion caused by heat. The appearances presented by the 
arc in air, hydrogen, and carbonic acid are very beautiful, and 
are copiously illustrated by copies, in mezzotint, of photographs 
and drawings. The authors conclude that for all gases there is 
a minimum pressure which offers the least resistance to the 
passage of an eleCtric discharge. After the minimum has been 
reached, the resistance to a discharge rapidly increases as the 
pressure of the medium decreases. That the potential necessary 
to produce a discharge between parallel flat surfaces at a constant 
distance and various pressures, or at a constant pressure and 
various distances, may be represented by hyperbolic curves. 
The resistance of the discharge between parallel flat surfaces 
being as the number of molecules intervening between them. 
That the eleCtric arc and the stratified discharge in vacuum 
tubes are modifications of the same phenomenon. 
“ Preliminary Note on Magnetic Circuits in Dynamo- and 
Magneto-EleCtric Machines,” by Lord Elphinstone and Charles 
W. Vincent, F.R.S.E., F.C.S., F.I.C. The experiments which 
form the subjeCt of this note were made in connexion with an in- 
vestigation as to the best form for the construction of a dynamo- 
eleCtric machine, intended to furnish currents of high intensity 
in great quantity. The principles deduced applies equally, how- 
ever, to magneto-eleCtric machines. The authors show that 
electro-magnets do not lose their magnetism, as commonly sup- 
posed, immediately on cessation of the voltaic current, but that 
if the poles are “ closed ” by approximation of other magnets or 
armatures, the magnetism is retained for an indefinite time. A 
magnet 581bs. weight could be lifted by its armature, and sparks 
obtained from the ends of the wires of its helix a week after 
magnetisation. The authors have constructed a dynamo-eleCtric 
machine, in which the “closed magnetic circuit” is utilised, 
which they propose to describe in a future paper. 
“ Preliminary Report to the Committee on Solar Physics on 
the evidence in favour of the Existence of certain Short Periods 
common to Solar and Terrestrial Phenomena,” by Balfour Stew- 
art, F.R.S., and William Dodgson. In a previous Report to this 
Committee the authors exhibited a method of detecting the un- 
known inequalities of a series of observations, and gave some 
evidence that the temperature-range and the declination-range 
at Kew Observatory are both subject to the same short-period 
inequalities, the particular periods investigated being those 
around twenty-four days. In the present paper they investigate, 
after the method described in their first Report, the following 
ranges : — 
I. Diurnal Ranges of Temperature of Air at — a Kew (al- 
ready given) ; Toronto ; y Utrecht. 
II. Diurnal Ranges of Magnetic Declination, including Dis- 
turbances at — « Kew ; Prague. 
