halley's comet: physical observations. 89 



Observations were taken every five minutes, as the instru- 

 ment was usually charged with negative electricity. The rate 

 of leak on the morning of the i8t"h was nearly twice as great 

 as that on other days, and this irregularitv' was not again 

 observed and not to anything like the same extent. As far 

 as I know at present the comet's tail could not have been 

 near the earth at this time, and so the increased rate of leak 

 must have been due to some other cause. 



It may be asked: " What electrical disturbances might have 

 been expected?" To this question no very precise answer 

 can be given, but it was not unreasonable to suppose that if 

 the tail consisted of fine particles driven by some repulsive 

 force from the sun, such as the radiation pressure, these 

 particles would have been electrified, and on entering the 

 atmosphere, or even if they approached near to the earth, 

 would have exerted some influence on the electrostatic field 

 surrounding the earth; or if they actuallv entered the atmos- 

 phere would have had some influence on it's conductivity. From 

 the observations it seems clear that no such effect was pro- 

 duced at the time of the passing of the comet. 



(3) Magnetic Observations.— -It was at first intended to take 

 observations of declinations and dip, and from there to deduce 

 the total force, but owing to the non-arrival of apparatus, the 

 decimation only could be taken. The apparatus used was a 

 Kew pattern Magnetometer tested at the National Physical 

 Laboratory. This was set up in an underground room in 

 which the temperature was very constant. Observations were 

 taken for three weeks, and, with one slight exception, showed 

 no unusual variation. There was an increase in the declina- 

 tion at 10.30 on the night of the i8th. 



Table III. gives the value of the declination in terms of 

 the position occupied by the cross wire on the scale of the in- 

 strument, as the variation only, and not absolute value was 

 required. The maxima occurred at about 10 a.m. and 10 p.m. 

 but on the night of the i8th the maximum was at 11. 10. The 

 change was, however, of a very small order, and not more 

 than would have been caused by a slight megnetic storm. 

 The maxima occurred at about the usuaftime on the morning 

 of the 19th. The magnetic observations recorded in the 

 following table represent the period from 10 p.m. on 18th 

 to T2.30 p.m. on 19th Mav. 



The value of i scale division of the instrument is 58". 



Table III. 



