531 
1909-10.] Halleys Comet and the Earth’s Atmosphere. 
a little more to the south, and the numbers began to rise to from 1000 to 
2700 per c.c. On the 21st there was no well-marked general circulation, 
and the local wind was from the west and light, and the numbers fell to 
from 341 to 1750. On the 22nd the general circulation was still weak and 
the numbers were slightly higher. On the 23rd, 24th, and 25th the general 
circulation was feeble and irregular, and the number of particles varied, 
being as low as 626 on the 23rd, rose to 5500 on the 24th, and remained 
between 3000 and 4000 on the 25th. On the 26th the wind changed to 
S.S.W. in the morning, but veered to N.W. in the evening, and the number 
of particles fell as low as 203 per c.c. During the next day the numbers, 
under the influence of the west wind, varied from 70 to 560. During the 
28th, 29th, and 30th the numbers varied from 217 to 1500, the general 
circulation and local winds being westerly. 
The question now is, — How do these figures compare with the previous 
observations made at Kingairloch ? East winds at Kingairloch have been 
observed with as many as 3000, 4000, and even 5000 particles per c.c., and 
S.E. winds have also been observed with similar numbers, so that the 2750 
observed in S.E. winds at Morar on the 20th is only what might be expected. 
There is nothing unusual in the figures for the 21st, 22nd, and 23rd. But 
when we come to examine the figures for the 24th and 25th, which are 
much higher than previously observed at Kingairloch in W. and N.W. 
winds, we are presented with a puzzle which may be interpreted in different 
ways, according to the view one feels inclined to adopt. It may be comet’s 
dust or it may be due to the meteorological conditions. During these two 
days the centre of an irregular and feeble anticyclone was situated over 
Morar, and there was no general circulation. The meteorological charts 
show light winds blowing in different directions over a limited area, and 
it has been shown in previous communications that these are the conditions 
in which large numbers of particles may be observed, as local impurities 
tend to increase under these conditions. This explanation, however, does 
not seem very satisfactory, because in that district local pollution is very 
small, and, further, the centre of the anticyclone on the 24th was situated 
partly over the very north-west of Scotland, but mostly over the sea to the 
north ; so that, under the conditions, local pollution hardly seems sufficient 
to account for the high numbers. 
On the other hand, if we wish to blame the comet for these high numbers, 
then we can point out that it would be just in such a position that the 
comet’s dust could be first observed, as it would reach the earth’s surface 
quickest in the descending current in the centre of an anticyclone, as the 
movement of these fine dust particles is more determined by air currents 
