5/8 



SCIENTIFIC NEWS. 



[Dec. 7, if 



with instruments of moderate dimensions. To many 

 persons the cost and difficulty of construction of great 

 telescopes seems out of proportion to the optical advan- 

 tage gained, but the same thing is seen in other depart- 

 ments of astronomy, as well as outside of the science. A 

 sextant, with which the places of the stars can be 

 determined to within a fraction of a minute of arc, costs 

 less than a hundred dollars, while thousands must be 

 expended if fractions of seconds are to be taken into 

 account, the error of position in either case being beyond 

 detection with the unassisted eye. The 12-inch tele- 

 scope has been used by Mr. Barnard for the observation 

 of comets and nebula;. It has been found by him to be 

 capable of giving photographic images of exquisite sharp- 

 ness, and in this capacity forms an important addition 

 to the outfit of the Observatory. Twenty-five new 

 nebula? have been discovered by Mr. Barnard with this 

 telescope, and a comet (comet e, 1888) was discovered 

 by the same observer with the 4-inch comet seeker, on 

 September 2nd. It is probable that the 12-inch telescope 

 will be fitted with a new driving clock, in order to better 

 fit it for photographic work. No change has been made 

 in the dome and hydraulic elevating floor of the large 

 telescope. The convenience, and indeed necessity, of the 

 elevating floor is every day more apparent. The rapid 

 motion of the eye end of the telescope (a foot in eight 

 minutes for an equatorial star) would alone make the use 

 of an observing ladder proportioned to the size of the 

 instrument, extremely troublesome. The pier, when 

 finally placed in exact position, will probably be filled 

 with brick and sand. The driving clock of the large 

 telescope was provided by the makers with an electric 

 control for keeping its rate in exact coincidence with that 

 of a standard astronomical clock. The vertical shaft of 

 the governor rotates in one second, and has near the 

 bottom a small projecting pin. A stud on the armature 

 lever of an electro-magnet is struck by the pin as the 

 governor shaft rotates, when a current is passing through 

 the magnet, but when the current is broken once a second 

 by a standard clock, the stud is withdrawn at the proper 

 instant to allow the pin to pass. There is also an 

 ingenious and beautifully constructed attachment for 

 breaking the circuit in case the standard clock should, 

 either by accident or design, omit one or more seconds 

 in a minute. The driving clock is adjusted to run a little 

 fast, and is continually checked by the control, the 

 governor being allowed to rotate by turning on a friction 

 collar. It was found, however, that the impact of the 

 pin on the governor shaft against the stud of the arma- 

 ture caused a shock which was transmitted to the telescope, 

 and produced a disturbance of the image fatal to photo- 

 graphic work. The control was therefore removed, and 

 another which Mr. Keller devised for the purpose of giving 

 a perfectly smooth motion, was substituted for it. The new 

 control answers its purpose so well, and is of such 

 extreme simplicity, that we shall give a description of it 

 here, as it can be applied to any clockwork having a shaft 

 which rotates in an integral part of a second. A soft iron 

 sector, subtending an angle of 36°, and having a radius 

 of six inches, is clamped to the vertical axis of the 

 governor, and rotates in a horizontal plane. The sector 

 passes very close to the poles of an electro-magnet (part 

 of the old control) which is mounted on a slightly elastic 

 standard of steel. At every second a strong current is 

 sent through the coils of this magnet, by means of a 

 standard clock, the circuit being closed, as in the case of 

 the old control, by the relay points of the chronograph 



attached to the driving clock. The driving clock is set 

 so as to run a little too fast, and when the governor is 

 started the sector gradually gains upon the click of the 

 chronograph until it reaches the magnet of the control, 

 when the friction produced by the attraction of the latter 

 prevents any further acceleration, and the governor will 

 rotate in exactly one second by the standard clock, as 

 long as the control is in operation. 



The elasticity of the support on which the electro- 

 magnet is mounted plays an important part in the 

 proper working of the control. When the sector passes 

 at the exact instant of the passage of the current, the 

 magnet springs in toward the sector, and comes into 

 actual contact with it, very greatly increasing the friction, 

 while the passage of the sector at any other instant 

 meets with no resistance, the magnet bei--~~ 1,v Mlv with- 

 drawn by its support. The current used -trol 

 is obtained from the battery of twen , 

 employed during the daytime in transmitting time signals 

 to San Jose. As the signals are not sent at night, the 

 battery is then connected with the control by turning a 

 switch. With this control no shock is communicated to' 

 the telescope ; and the image of a star is steady. Since, 

 however, changes of refraction and slight irregularities in 

 the clockwork produced small displacements of the 

 image in a telescope, it has always been necessary in 

 photographing with long exposures to keep the telescope 

 pointed by hand, correcting any displacement which may 

 occur by the slow motions of the instrument. It was 

 found impracticable to move the immense mass of the 

 Lick telescope with the quickness and delicacy required 

 in this operation, and after various experiments, Mr. 

 Schaeberle suggested that the photographic plate should 

 be mounted upon double slides, one moving in right 

 ascension, and the other in declination, and should be 

 kept upon a star by means of a diagonal microscope 

 attached to the plate. A rough experimental model was 

 constructed on this plan by the observatory machinist, 

 and performed so satisfactorily that a plate holder of 

 more accurate workmanship will be made on the same 

 principle. The public receptions on Saturday evenings 

 interfere greatly with these experiments, as all apparatus 

 must then be removed to fit the telescope for visual 

 observation. Probably few visitors are aware of the 

 hindrance to astronomical work caused by their enter- 

 tainment, although, as a duty to the public the sacrifice 

 is always cheerfully made. Many fine nights are to be 

 expected during the months of October and November, 

 but after that, fog and rain will almost put an end to 

 observation until the succeeding spring. 



Wolverhampton Literary and Scientific Society. — 

 On November 20th Mr. Harold B. Dixon, F.R.S., de- 

 livered a lecture on " The Nature of Explosions," before 

 this society. Mr. Dixon demonstrated the different kinds 

 of explosion produced by gases, gunpowder, and gun- 

 cotton. He referred to the difference in the mechanical 

 means by which the oxygen and the combustible body 

 come together, and said that explosions, according to 

 two distinguished French chemists, could be divided 

 into three parts — the ordinary combustion, the inter- 

 mediate period of vibration, and the period of detonation. 

 Reference was made to the measurements of the ex- 

 plosions of gases, and the conclusions arrived at under 

 this head, together with illustrations of the amount of 

 pressure produced in instantaneous explosions. 



