THE RETURN OF HALLEY’S comet In 1910. 31 
are, however, difficulties in the way, for a volume of gas has 
no rigidity, and cannot rotate as a whole; even if we admit a 
controlling force, each molecule of the tail would rotate with 
a different period, according to its distance from the axis. It 
seems to me that a rotation of the head would produce a 
semblance of rotation in the tail emitted by it. The structure 
in this case would be spiral, a form suggested by some of the 
photographs of Morehouse’s comet of 1908. On its emergence 
from the sun’s rays in 1856, Halley’s comet was best placed 
for southern observers, and Sir J. Herschel and Maclear at the 
Cape made drawings of it. It seems to have lost its tail in 
January, two months after perihelion, so we must be prepared 
for a similar phenomenon in June next. Morehouse’s comet, 
in like manner, went through a tailless phase several times 
during its period of visibility. It appears that all predictions 
as to the brilliance of a comet at any particular time are quite 
uncertain; we can predict its distance from sun and earth, 
but not these physical changes, to which some comets seem to 
be much more subject than others. 
We have the great advantage of photography during the 
present apparition for giving a continuous and reliable record 
of all the variations, as also for enabling the comet to be 
detected eight months before perihelion passage, at which time 
it was of the sixteenth magnitude, that is, it only gave zo¢o5 
of the light of a sixth magnitude star, which itself is barely 
visible to the unaided eye. For the first few months the 
comet brightened up rapidly, and by mid-November it was an 
easy object in telescopes of moderate size, being as bright as 
the tenth magnitude. Then the increase seemed to be arrested, 
and it only increased very slehtly in brightness up to the 
middle of January; but by the end of that month there were 
‘evident signs of a tail forming, and by mid-February the comet 
was seen by the naked eye by Professor Wolf, who had also 
been the first to detect its presence on the photographic plate. 
A photograph taken at Juvisy showed quite a conspicuous 
tail, and a drawing at the beginning of March showed a 
remarkable double tail, not unlike some of the sketches made 
in 1835. The comet was then lost in the sunlight, but 
reappeared as a morning star about the middle of April. It 
had greatly brightened during its absence, and was now of the 
second magnitude. At 4.30 am.on April 20th it passed its 
perihelion, and commenced another revolution, which will not 
be completed till early in 1986. I have myself seen it early 
in May. when it was quite conspicuous, in spite of its being 
