416 SECTIONAL TRANSACTIONS.—A. 
is placed between the poles of a powerful electro-magnet. This detector consists 
of a short loop of fine platinoid wire, under tension, and spanned by a small 
plane mirror. The clock signal or other signal being given as an electric current 
of a few milliamperes is recorded photographically on a moving film by help 
of a minute rotation of the mirror. The rate of motion of the film cannot be 
trusted for fine measurement over an interval of one second; therefore the 
standard is taken from an interrupter which cuts off the light falling on the 
«irror at intervals of approximately 0.1 sec. This interrupter is a simple steel 
tongue, set in vibration and not maintained in any way, and carrying a wire 
which occults a slit through which the light passes. 
The scale employed can be varied within wide limits. That used most 
frequently at present gives a motion of the film of about 3:3 cm. per second, 
with a lateral displacement of 0.13 mm. per milliamp., corresponding to a 
magnification of the movement of the wire loop of the detector by about 1,000. 
Exhaustive tests have shown that the instrument is completely reliable. 
29. Rev. A. L. Cortre.—The Magnetic Storms of the Present Solar 
Cycle. 
A comparative table of magnetic and solar phenomena, including sun-spots, 
bright prominences, H, absorption markings, and calcium flocculi, has been 
constructed for the years 1913-1921 (June) from the records of the Stonyhurst, 
the Tortosa, and the Kodaikanal Observatories. Sunspots attained their 
their maximum in 1917; bright prominences théir maximum profile-area in 1915 
and in 1917; bright calcium flocculi and dark H , absorption markings their 
maximum area in 1920. ‘The greatest number of magnetic disturbances, g., and 
v.g., took place in 1919. A g. disturbance is one in which the range exceeds 
the mean daily range of the five quietest days, in D and H combined, by 15’. If 
the excess is 20' and over it is marked v.g. 
1. The general magnetic activity in this solar cycle would appear to coincide 
with the mean daily area of the calcium flocculi, and the hydrogen absorption 
markings. 
2. Consistently with the results for past solar cycles, the magnetic phenomena 
increase in activity, as the sun-spots and the flocculi decrease in mean latitude, 
after the sun-spot maximum has been attained. This is due to the fact that, 
on the whole, disturbed areas on the sun are more effective when they are 
situated in or near the heliographic latitude of the earth. 
3. There have been four outstanding magnetic storms of great violence 
during the present cycle: (a) 1917, August 9, 10, renewed on August 13, 14; 
(6) 1919, August 11, 12; (c) 1920, March 22, 23; (d) 1921, May 12-21. In the 
cases of (a), (c), and (d) the magnetic storms were coincident with the passage 
of very great sun-spots across the sun’s disc. The most active period of sun- 
spots was 1917, August 6-16, and a big active group, lat. +16°, maximum area 
3.444 on August 9, was near the central meridian when the series of magnetic 
disturbances began. In 1920 there was but one greatly disturbed area, mean 
latitude —6°, which extended in longitude 36°, and which was active from 
1919, December 27, to 1920, May 16. Its maximum area was 3,652 on March 23, 
when a series of synodical magnetic disturbances culminated in a violent storm. 
it was also near the central meridian, and in latitude — 5°. Similarly the 
protracted storm of 1921 coincided in its maximum phase with the central 
meridian passage of a very large sun-spot group, area 3,300, on May 14. 
These cases support the view that the action of a sun-spot area, as the 
necessary condition of a magnetic storm, is by the projection of a set of 
divergent rays proceeding fan-wise from the area, and probably diffused into 
a cloud-belt. For some of the series of magnetic storms, accompanying the 
passage of the sun-spot area, occur when the sun-spot is far removed from the 
central meridian. 
But with regard to the violent magnetic storm of 1919, August 11, 12. the 
largest sun-spot group of the year, a triple equatorial group, extending 20° in 
longitude, was approaching the sun’s E limb on August 11. This very large 
group formed a procession with another, spectroscopically very active, group 
that had preceded it, so that the same region of the sun was disturbed from 
August 4 to August 24. The violent magnetic storm might have been con- 
nected with the greatly disturbed area which passed the central meridian 
ee ae 
