March 12, 1885] 



NA TURE 



439 



Here the new mechanism comes in. This vertical axis 

 supports a sort of bracket, P, to the extremity of which is 

 attached a small bead or ball, B, by means of a thread a few 

 centimetres long. Putting out of view meantime the other 

 parts resting on the case, it will be seen that the axis, by 

 the action of the main spring, will turn with a rapid 

 movement, drawing the ball B along with it. To regulate 

 this movement, it is sought to interpose in its path suit- 

 able obstacles ; this is the object of the horizontal wire 

 terminating in the hooks T, and of the vertical pillars fixed 

 on the case. The bracket P draws the thread in its 

 movement and makes it strike against the arm T ; it is 

 thus arrested, and by virtue of its acquired speed, the ball 

 B winds the thread around the pillar on the left ; then 

 follows an unwinding of the thread and a rewinding in an 

 inverse direction, which enables the thread to pass the 

 point T. But in unwinding it strikes a second time 

 against the pillar, winds and unwinds anew, and only 



succeeds in passing this double obstacle after four suc- 

 cessive windings, twice in one direction, and twice in the 

 opposite direction around the same pillar. The thread 

 thus set at liberty permits the bracket to turn 180 around 

 the vertical axis. After this rotation it encounters two 

 analogous objects placed on the right of the clock, and is 

 delayed a certain time before passing these objects and 

 returning to the pillar on the right. By suitably varying 

 the length of the thread, which is easily done by means 

 of a runner on the bracket, we obtain the complete phase 

 of the movement with its eight successive windings of the 

 thread, lasting exactly six seconds ; and the clock is thus 

 regulated, if not with all the precision of a chronometer, 

 with an approximation said to be sufficient for ordinary 

 use. The principle applied in this clock might possibly 

 be utilised in cases where it is sought to regulate a slow 

 movement of rotation by simple arrangements, both 

 economical and uncumbrous. 



A CLOUD-GLOW APPARATUS 



"D Y the kindness of Prof. J. Kiessling, of Hamburg, we 

 *-* illustrate a simple and easily arranged piece of 

 apparatus which he has designed for the purpose of ex- 

 hibiting on an experimental scale some of the many 

 colour-phenomena which are produced when direct sun- 

 light, or electric light, penetrates a moist or a dry cloud. 

 In particular the apparatus can be used to produce on an 

 artificially excited mist the same kinds of intense colora- 

 tions which were visible in such extraordinary brilliancy 

 in the winter 1SS3-S4 during the hours of twilight at 

 almost ever)- place the whole world over. 



The following pieces compose the apparatus : — 



I. A glass globe, A, Fig. 1, holding about 20 litres, fixed 



in a wooden support, and closed by a rubber stopper bored 

 with two holes. Through these holes enter two tubes of 

 glass (1) and (2), with taps ground in. 



2. An air-filter, c, consisting of a glass tube 30 centi- 

 metres long, filled with cotton-wool. 



3. An india-rubber pump, B, for producing spray or 

 mist. This is simply part of a common spray-apparatus, 

 and is set so that it draws air from the air-filter and 

 delivers it into the globe. By this means a pressure of 

 one-sixth to one-fifth of an atmosphere is readily obtained. 

 Suppose 15 or 20 grammes of water to have been intro- 

 duced into the globe and such a pressure to have been 

 produced, and then after about 10-15 seconds the other 

 tap (2) to be suddenly opened, or removed quite out of 

 the tube, the release of pressure will result in a sudden 



lowering of the temperature, and the production of a 

 tolerably homogeneous mist, the density of which will 

 depend on the quantity of aqueous vapour present. 



4. A simple heliostat, E, consisting of a mirror capable 

 of being turned either in altitude or azimuth on an iron 

 stand, and also of being clamped at any desired height. 



5. A Woulff's wash-bottle, d. This can be filled with 

 hot water so as to yield a supersaturated atmosphere ; or, 

 by the addition of ammonia or of hydrochloric acid, may 

 furnish vapours of these materials for experiment in the 

 globe. 



6. A cylindrical tin-ware vessel, F, with a spherical 

 bottom, to be set upon the glass globe, to heat or cool it 

 as may be desired. 



The following experiments may be made with this 

 apparatus : — 



[1] The Ordinary Lunar Halo. First cover the surface 

 of the mirror with a card disk having a central circular 

 opening 2 centimetres in diameter, covered with tissue 

 paper. In direct sunlight observe the bright surface of 

 this circle of tissue paper (which serves as an artificial 

 moon) through the mist that is produced in the globe by 

 letting in a stream of moist vapour from a flask of hot 

 water for a few seconds. The halo is yellowish with a 

 red-brown edge. 



[2] Blue Sun. Pour into the globe a little hydrochloric 

 acid, and blow in air through the wash-bottle, having 

 filled the latter first with liquid ammonia. A dust-cloud 

 of fine particles of sal-ammoniac is thereby produced. A 

 ray of direct sunlight viewed through this is curiously 

 coloured, appearing at the first moment red, and then 

 changing to bluish violet and to full blue. 



