[THOMSON] PASSAGE OF HALLEY’S COMET 63 
The observations were begun on the morning of May 18th, and 
were continued without interruption until noon on May 21st. The 
results of these observations expressed as the number of ions made per 
cubic centimetre per second in the ionisation chamber are represented 
by a curve shewn in Fig. 1. From the numbers given it will be seen 
that while the normal ionisation was about 30 ions per ce. per second 
two rather remarkable increases were noted in the observed ionisations. 
One of these occurred on May 19th, beginning at 1 o’clock p.m., and 
lasting until about 4 o’clock p.m., and the other on May 21st, commencing 
at about half past nine in the morning and continuing for a period of 
about one hour and a half. On these two occasions it will be seen that 
the ionisation rose to about 100 and 130 ions per cc. per second respect- 
ively. 
Whether these unusual increases in the ionisation were due to the 
presence of the comet or not, it appears to the writer impossible to 
decide. A. Wigand' in a paper on observations made at Halle from 
May 17 to May 20, records decided diminutions in the earth’s horizontal 
magnetic field during the early hours of May 19 and again from 7.30 to 
11.30 a.m., of the same day. He also records a marked increase in the 
electrical conductivity of the atmospheric air between the hours of five 
and nine o’clock on the morning of May 19 and at the same time an 
anomalous depression in the potential gradient. Wigand concludes 
from his observations that these magnetic and electrical disturbances 
were in all probability connected with the passage of the comet. 
The disturbances noted by Wigand it will be seen took place on 
May 19th at 11 a.m., .e., about 12 hours before the one at Toronto 
occurred. The phenomena observed by him were the electrical “ dissi- 
pation” and the potential gradient, while that investigated by the 
writer was the ionisation in the gas within an hermetically closed 
metallic vessel. It is possible to conceive that three such phenomena 
could be related but in the present case it is difficult to see any connec- 
tion between the two sets of disturbances noted and still more difficult 
to establish a connection between them and the passage of the comet. 
To the writer there appears to be four possible explanations of 
the disturbance noted by him. It could in the first place be caused by 
bringing radioactive bodies into the neighbourhood of the ionising 
chamber. It might also be produced by temporary faulty instrumental 
conditions or adjustments. Then again the disturbance might be the 
result of some intense temporary solar activity or finally it might be 
due to the passage of the comet. The first explanation can be easily 
dismissed for no radioactive bodies were brought near at any time while 

1 Wigand, Ber der Deut. Phys. Ges. Heft 13, p. 511, 1910. 
