[THOMSON] PASSAGE OF HALLEY’S COMET À 65 
the observations were being taken. In regard to the second explanation 
it may be stated that extreme care was taken to perfect the conditions 
of measurement. Even after the measurements were finished the 
whole instrumental outfit was again closely scrutinised but a rigid 
examination failed to reveal any faulty adjustment. 
The writer is therefore inclined to the view that the disturbance 
was not due to faulty conditions of experiment or to faulty adjustments. 
With the object of corroborating this view observations were again 
made for periods of about twelve hours on May 23, 24, 25 and 26. The 
results of these are illustrated by a curve in Fig. II. These observations 
it will be seen failed to shew anything abnormal. The ionisation varied 
between narrow limits and the observations were practically of the 
same character as those made by Wright and Cline on previous occa- 
sions. 
If the disturbances noted were due to faulty adjustments or to 
some cause associated with the sun’s activity one would expect a repeti- 
tion of the disturbance. None such occurred, however, during any of 
the four later periods of observation, and this absence of a repetition 
would go to confirm the view that the disturbance was not due to any 
defect in the measuring apparatus. 
The absence of a repetition on the four days mentioned, however, 
would not entirely exclude solar activity as a cause of the disturbance. 
To establish this point it would be necessary to make a more extended 
series of observations. 
All the readings recorded in the present paper were taken visually 
but the strain of making measurements in this manner over long con- 
tinued periods of time is too great for visual observation to be practic- 
able and the writer therefore hopes to continue, at an early date, the 
observations with self recording apparatus with a view to seeing whether 
solar activity is ever really accompanied by such terrestrial disturbances 
as those noted or not. 
This information would be useful in deciding between solar activity 
and the comet’s influence as the cause of the disturbance. 
In conclusion the writer wishes to acknowledge his indebtedness 
to Professor McLennan for suggestions during the investigation, and 
for his kindness in scrutinising the installation of the apparatus and also 
to Mr. W. T. Kennedy for his kind assistance in taking some of the 
readings. 
Sec. IIT., 1910. 5. 
