20 



NATURE 



[March 3, 1910 



number of the Comptes rendits. He remarks that the 

 tail was always bifurcated, as though the nucleus formed 

 3. screen to the repulsive force acting on the vapours, and 

 at its greatest length extended to a distance of 50°. Photo- 

 graphs taken at the Fabra Observatory show the details 

 very well, but are not suitable for reproduction. Special 

 attention was paid to the secondary tail on the left of 

 the comet, which was much fainter than the primary, and 

 extended to the opposite side, that is, towards the sun. 

 This tail was concave to the direction of the motion, and 

 it is this point that M. Sola discusses. He attributes the 

 concave form to the differential action of the gravitational 

 forces of the sun and the nucleus and the repulsive action 

 emanating from the sun. The greater number of particles 

 are repelled and form the principal tail, but others, depend- 

 ing upon their nature, will not be driven away entirely, 

 and will become attracted towards the sun. The finest of 

 these will be disseminated into space, but the heavier ones 

 will, in falling towards the sun, be attracted by the 

 nucleus, and thus produce the observed form concave 

 relatively to the comet's motion; those falling into the 

 nucleus will be again expelled, but others of which the 

 velocity sunwards is too great will rush past the nucleus 

 and form the fan seen on the side of the head nearer the 

 ■sun. This hypothesis admits of two tests : — (i) before 

 perihelion the concavity should not precede the nucleus ; 

 (2) spectroscopic investigation should show the two tails, 

 at a distance from the nucleus, to differ chemically. M. 

 Sola gives a formula by which the acceleration of the 

 different particles might be computed on the assumption 

 that the forces he assumes are all active, but the photo- 

 graphs of comet 1910a do not permit of its application. 



Fireball of February 17. — Mr. W. F. Denning 

 writes : — " A considerable number of reports have been 

 received concerning the appearance of this brilliant object. 

 It was seen so far east as in Essex and Kent, and observers 

 In Gloucestershire, Somerset, Devon, and Glamorgan 

 witnessed the descent of the fireball under a more brilliant 

 aspect. The long endurance of the trail was one of the 

 most noteworthy features, and reminds us of the remark- 

 able meteor of February 22, 1909, which left a streak for 

 three hours. 



From the best observations it appears that the fireball 

 of February 17 last descended in a nearly vertical path 

 over a point about twenty miles north-west of Land's 

 End, Cornwall. That its height was approximately 

 eighty-eight to forty-six miles appears very probable, but 

 observations would be valuable from Cornwall and the 

 ■south coast of Ireland. 



"To have created so brilliant an effect in the evening 

 twilight the meteor must have been one of the very largest 

 type, and the shower in Auriga from which it was pre- 

 sumably directed is one of extremely interesting character. 

 Meteors of great size and startling brilliancy are more 

 frequent in the early evening than at any other period of 

 the night, and this is a circumstance to which the writer 

 has several times directed attention." 



It is reported that a brilliant meteor, as bright as Venus, 

 was seen from many parts of Yorkshire soon after sunset 

 on Tuesday last, March i. 



Halley's Comet. — In an article on Halley's comet 

 •which appeared in the Times of February 24, the writer 

 states that the comet is likely to be at its best, as a dis- 

 play, during the last ten days of May, when it will be an 

 ■evening star and near to the earth. He also suggests 

 that between March 11 and April 24 the comet will prob- 

 ably be unobservable, being too near the sun as seen from 

 the earth. On the latter date it should become visible as 

 .1 morning star, rising just before dawn ; but while its 

 Intrinsic light will then be nearly at its maximum, its 

 apparent brilliancy will suffer on account of the distance 

 Trom the earth. After the transit on May 18, the comet 

 will again become an evening star, and, being then much 

 nearer the earth, should be seen at its best. During the 

 transit Mr. Evershed is hoping to employ both heliograph 

 and spectroheliograph at the Kodaikanal Observatory to 

 obtain records of the comet's nucleus. That this contains 

 a large amount of solid matter is rendered probable by 

 the endurance of the comet for at least 2000 years ; simple 

 vapour would scarcely hold together for so long, and 

 NO. 2105, VOL. SlII 



could not be expected to disseminate and lose entirely tho 

 amount of matter that has been poured forth by Halley'-- 

 comet even during its known career. A photograph taken 

 at Greenwich Observatory on January 30 showed a fairly 

 defined nucleus surrounded by a large diffused coma, and 

 a very faint tail. 



PiDOUx's Comet, jgiob. — No further official news of the 

 comet discovered by M. Pidoux at Geneva on February 20 

 is to hand, nor had any further observations been reported 

 to the Astronomische Nachrichten up to February 25 ; but 

 according to a Geneva correspondent of the Daily Chronicle 

 M. Pidoux discovered the new comet photographically 

 whilst photographing Halley's comet, and confirmed the 

 discovery visually. He says the new comet is V-shaped, 

 and travelling at a great speed in a south-west direction ; 

 also that it is brightening, and, given good weather, should 

 become a naked-eye object in a few days. No magnitude 

 is mentioned. 



A Naked-eye Sun-spot Group. — A remarkable group of 

 spots has been observed on the solar disc during the past 

 fortnight. Following a pair of spots of moderate size, a 

 small spot was seen to come round the eastern limb on 

 February 17, and on February 19 developed into a group. 

 This group increased in numbers and extent until, on 

 February 23, the length of the affected area, measured 

 along the greatest diameter, was about one-seventh of the 

 solar diameter, roughly 120,000 miles. On March i most 

 of the group had passed over the western limb, and there 

 was a low bank of prominence matter, seen in the C line| 

 of hydrogen, lying above its position on the limb. The; 

 visual observations at the Soiar Physics Observatory, South' 

 Kensington, showed that a little to the north of the spot 

 disturbance there was a bright trifurcated prominence on 

 the limb, which changed its form considerably between 

 10 a.m. and i p.m. Mr. J. H. Elgie reports having seen 

 the above spot group with the naked eye on February 25. 



THE BR EN NAN MONO-RAIL SYSTEM. 



A LARGE company of engineers and others interested, 

 ■'*• including representatives of the .Admiralty and of the 

 War Office, assembled at the Brennan Torpedo Works, 

 near Chatham, on Friday, February 25, to witness a 

 demonstration of Mr. Brennan 's mono-rail vehicle. The 

 vehicle shown, which is the first of its kind, has been 

 designed for rough military purposes, not for high speeds, 

 and the trials on this occasion were intended to. show its 

 adaptability for this kind of work. - A general description 

 of the car and- the experimental track appeared on p. 79 

 of our issue for November 18, 1909, to which we may refer 

 our readers,, supplemented with the illustration now shown 

 of the vehicle standing on one of the sharp curves. In 

 this illustration may be noticed the radiators for cooling 

 the circulating water required for the petrol motors, these 

 radiators being secured to the front of the machinery cab; 

 the front boi'ie with its two wheels, the rear wheel being 

 driven by side rods and balanced cranks from one of the 

 two electromotors ; also one of the side chocks, on which 

 the car may rest Vv'hen required for unloading or other 

 purposes. There is a chock on each side of the car. Mr, 

 Brennan is the centre figure in the machinery cab. 



The car first made its appearance from the pier, carry- 

 ing a number of large packing cases and three or four 

 men, and was brought to rest. Then, running on to the 

 circular track of 105 feet radius, the speed was gradually 

 accelerated to twenty miles per hour, the car inclining 

 inwards automatically so as to counteract the effect of 

 centrifugal force. It is of interest to note that the load 

 was simply laid on the flat platform of the car, without 

 being secured in any wa}', and that there was not the 

 slightest tendency to disturb the position of any of the 

 packing cases while on the curve, thus showing the per- 

 fect balance maintained by the gyroscopes. While 

 stopping on the curve, the angle of heel gradually 

 diminished, and the car platform was level on rest being 

 attained. 



The operation of unloading in the field was then shown. 

 While the normal action of the gyroscopes is to maintain 



