1889 - 90 .] Mr J. Aitken on Dust Particles. 245 
risen since coal has come into such extensive use; or is the tempera- 
ture of a place higher when the wind brings dust-laden air to it 
from a populous district than when it brings pure air ; or has the 
temperature in the populous parts of New Zealand or Australia 
increased with the increase of population 1 Unfortunately this is a 
case where, wdiile a positive reply might support the conclusion, a 
negative one does not weaken it, because the dust from human 
habitations keeps near the ground, and if there is no protection 
above it, its effect will probably be, not to keep up the night 
temperature, but rather to lower it, on account of the increased 
radiating power of the atmosphere caused by the dust. 
There is still another reason why we must move herewith caution. 
Should the possibility here shadowed fortldbecome a reality, then we 
should find it entering the arena of one of the most highly contested 
fields of scientific speculation — we should find it claiming to be the 
great regulator of the earth’s climate in bygone ages. Meteorolo- 
gists would be disputing with astronomers, and claiming for their 
domain the cause of the great changes which geologists tell us have 
taken place in the earth’s climate from time to time. They would 
no longer ask the astronomer if he could tilt the earth’s axis so as 
to bring first one part and then another under the influence of the 
sun’s rays. All the meteorologist would have to do would be in 
imagination to steer the solar system into a part of space where 
meteoric matter was absent. Were cosmic dust absent from the 
earth’s atmosphere, the earth’s heat would have a freer passage into 
space, and a glacial climate would be the result. To change this, 
it would only be necessary that the system passed to where meteoric 
matter was abundant, when the earth’s atmosphere, now full of 
cosmic dust, would conserve the sun’s rays and its own heat, and 
thus cause the glacial conditions to disappear. 
Such are some of the problems suggested by this investigation, 
none of which are worked out, and some only suggested. It is of 
course possible, perhaps probable, that many of the conclusions here 
set forth may be quite wrong. The whole phenomena are too con- 
plex to be solved so easily, and it will require continued records of 
all the meteorological phenomena, extending over long periods, and 
under many different conditions, before many of these conclusions 
can be satisfactorily established. 
