346 
POPULAE SCIENCE EEYIEW. 
equally well-marked periods of a large development of ozone. In February 
no such well-marked and settled periods occurred. On some days there was 
but little ozone, as, for instance, on the 2nd, and the afternoons of tbe 21st 
and 28th, especially on the afternoon of the 21st ; while on other days 
there was a good deal, as on the 16th, 18th, and 20th. But the periods of 
increased and decreased development were short and unsettled, often lasting 
not more than a few hours, and at most a day or two. In March there was 
scarcely a trace of ozone on the 1st and 2nd, and very little on the 3rd, 8th, 
9th, and 16th ; considerable quantities on the 13th, 25th, 27th, and 30th, 
especially on the 25th and 30th. Speaking generally, there was a period of 
scanty development of ozone till the middle of the month, and then a period 
of more plentiful development towards the end ; but neither of these periods 
was well defined or settled. — Vide Chemical News, April 19. 
The Measurement of the Magnetic Dip. — A paper was lately read before the 
Manchester Philosophical Society by Sir William Thomson, on a new form of 
the dynamic method for measuring the magnetic dip. Seven years ago, an 
apparatus was constructed for the natural philosophy class of the University 
of Glasgow, for illustrating the induction of electric currents by the motion 
of a conductor across the lines of terrestrial magnetic force. This instru- 
ment consisted of a large circular coil of many turns of fine copper wire, 
made to rotate by wheelwork about an axis, which can be set to positions 
inclined at all angles to the vertical. A fixed circle, parallel to the plane 
containing these positions, measured the angles between them. The ends 
of the coil were connected with fixed electrodes, so adjusted as to reverse 
the connections every time the plane of the coil passed through the position 
perpendicular to that plane. When in use, the instrument should be set as 
nearly as may be in the magnetic meridian. The fixed electrodes being 
joined to the two ends of a coil of a delicate galvanometer, a large deflection 
is observed when the axis of rotation forms any considerable angle with the 
line of magnetic dip. On first trying the instrument, Sir William perceived 
that its sensibility was such as to promise an extremely sensitive means for 
measuring the dip. Accordingly, soon after he had a small and more port- 
able instrument constructed for this special purpose ; but up to this time he 
had not given it any sufficient trial. On the occasion of a recent visit, Dr. 
Joule assisted at some experiments with this instrument. The results have 
convinced both observers that it will be quite practicable to improve it so 
that it may serve for a determination of the dip within a minute of angle. 
The Movement of Solids under Pressure. — M. Tresca, whose fine researches 
we have before noticed in these pages, has presented a second part of his 
memoirs to the Academy. This portion of his memoirs relates to the flowing 
of solids through simple or multiple circular, polygonal, and lateral orifices, 
and described a great number of interesting particulars which confirm 
the results of his former experiments, and undoubtedly prove that the mole- 
cules of substances of the stiffest nature in appearance possess the property 
of moving independently, generally in parallel directions. The flowing takes 
place in concentric zones. — Vide Comptes Rcndus, April 22. 
The Pei'iodical Variations of Tempei'ature. — M. St. Claire Deville has 
been publishing a second series of his investigations in the above interesting 
meteorological subject. He has established, in one of his former memoirs, 
