PART 



POLAR MAGNETIC PHENOMENA AND TERRF.U.A EXPERIMENTS. CHAP. V. 



643 



probably before the i8th May, as on that morning the main or northern tail got thinner as its distance 

 from the horizon increased (see sketches p. 15 of Transvaal Observatory Circular No. 3). 



"As to the actual and unbroken length of the tail, this was measured on the i7th May and found 

 to be 107. On the i8th the nucleus was invisible, but the tail ended 140 from the place of the head. 

 When the whole comet was visible the greatest length seen here was thus 107. I cannot find any 

 authentic measure of the angular length of a comet's tail which exceeded or was even as great as this, 

 but references to authorities are limited at this Observatory. It may be said that it would require much 

 imagination to desire a more impressive and brilliant spectacle than that presented by Halley's Comet 

 on the morning of the I5th, i6th, and I7th 

 Ma)'. It was indeed a Great Comet, such as 

 the writer had never seen before and can hardly 

 expect to see again. 



'The sketch given belove (fig. 238) may be 

 of use in following the records given in this and 

 tin previous circular. The tail on the 23rd May 

 and later dates proved that the comet's emissive 

 power had not lessened, and it will be remem- 

 bered that the tail of the 171(1 was still visible 

 in the morning sky in practically the same 

 position on the 2ist. From the i7th to the 2oth 



t may be assumed that the matter which would 



irdinarily go to form the tail accumulated in the 



riangle formed by the Earth and the comet's 



>'>sitions on the I7th and 2oth May; this matter 



>eing visible as the extensive glow involved 

 vith the Zodiacal Light. 



The lengths of the tails shown on the sketch are: 



-ZOMay 



23 May 



Fig. 238. 



1910 



Ma}- 1 7 

 20 

 23 



Units 

 0.30 

 0.09 



o.i 6 



Miles 



27 ooo ooo 



9 ooo ooo 



14 ooo ooo" 



The following is an account of a peculiar observation by EGINITIS(') on the evening of the zoth May. 



"On the evening of Friday, May 20, 1910, on looking at the head of Halley's comet through our 

 real equatorial Gautier (0.40 m.), we found it had completely changed its appearance since the last 

 bservation made in Athens (May 12); it was in the form of a crescent, resembling that of the moon a 

 ttle before its first quarter. The length of the axis of the head was about 2', almost four times less 

 lan its breadth; one would have said the comet had been truncated or partly occulted. The outline of 

 ie head towards the apex appeared very smooth and very bright, and was in the form of a parabolic 

 re, very luminous, not fringed externally, having its apex tangent interiorly to the nucleus. During the 

 bservation, this outline became smoother and smoother, while the tail, of which only a few traces, 

 :arcely more than the beginning, were visible in the concavity of the head, showed no perceptible 

 rolongation in the direction of the axis, unless it were a little at its margins (fig. 239). 



I'l "Ciel et Terre" XXXII, March 1911, p. 94. 



