Jan. II, 1877] 



NATURE 



227 



so that at each revolution of the radiometer a record is 

 printed on the strip of paper by dots ; close together if 

 the radiometer revolves quickly, farther apai't if it goes 

 slower. 



The power of the earth on the magnet is too great to 

 allow the radiometer to start without some initial im- 

 petus ; there should therefore be an astatic combination 

 inside the bulb, but for a single experiment it may be set 

 going by placing a i&w coils of insulated copper wire out- 

 side the bulb and depressing the battery key for an 

 instant. An electric current is thus passed through the 

 coils of wire, and the interior magnet is immediately 

 deflected from its north-south position ; the impetus thus 

 gained enables the light to keep up the rotation. 



For the purpose of measuring the amount of force 

 exerted by radiation I constructed a torsion balance 

 capable of indicating the millionth of a grain. A light 

 beam having two square inches of pith at one end is 

 _,alanced on a fine fibre of glass ^ stretched horizontally in 

 a tube, one end of the fibre being connected with a torsion 

 handle passing through the tube, and indicating angular 

 movements on a graduated circle. Th^ beam is cemented 

 to the torsion fibre, and the whole is enclosed in glass and 

 connected with the mercury pump by a spiral tube and 

 exhausted as perfectly as possible. A flat oblong piece of 

 soft iron weighing accurately o'oi grain is put into the 

 cross tube under the pith surface. This weight can be 

 picked up by a horse-shoe magnet outside the tube and 

 dropped on any part of the pith. A mark is made at the 

 exact ccnti-e of the pith surface, and by moving the mag- 

 net about it is easy to place the iron weight accurately on 

 this mark. A ray of light from a lamp reflected from a 

 mirror in the centre of the beam to a millimetre scale 

 four feet off shows the slightest movement. When the 

 reflected ray points to zero, a turn of the torsion handle 

 in one direction or the other will raise or depre:.s the pith 

 end of the beam, and thus cause the index ray to travel 

 along the scale to the right or to the left. If a small 

 weight is placed on one end so as to depress it, and the 

 torsion handle is then turned, the tendency of the glass 

 fibre to untwist itself will ultimately balance the downward 

 pressure of the weight, and will again bring the index ray 

 to zero. It was found that when the weight of the i-iooth 

 of a grain was placed on the pith surface the torsion 

 handle had to be turned twenty-seven revolutions and 

 353°, or 10073° before the beam became horizontal. The 

 downward pressure of the i-iooth of a grain was therefore 

 equivalent to the force of torsion of the glass thread when 



f twisted through 10073°. 

 I then found out the degree of delicacy of the balance. 

 1° of torsion gave a very decided movement of the index 

 ray, a torsion of 10073° balancing the i-icoth of a grain, 

 while 100074° overbalanced it. The balance will therefore 

 turn to the 99-1 00,000, oooth of a grain. 

 Weighed in this balance, the mechanical torce of a 

 candle 12 inches off was found to be 0*000444 grain ; of a 

 candle 6 inches off o'ooi772 grain. At half the distance 

 the weight of radiation should be four times, or o'ooi776 

 grain ; the difference between theory and experiment 

 being only four millionths of a grain is a sufficient proof 

 that the indications of this instrument follow rigidly the 

 law of inverse squares. An examination of the differences 

 between the separate observations and the mean shows 

 that my estimate of the sensitiveness of this balance is 

 not excessive, and that in practice it will safely indicate 

 the millionth of a grain. 



I performed an experiment at the meeting of the Royal 



Society on March 30 last to demonstrate the movement 



of the glass case of the radiometer. I made use of a 



^ large radiometer in a 4-inch bulb with ten arms, eight of 



' The torsion of fibre must be selected with great care. Ten threads were 

 i drawn out before the blowpipe and suspended from a horizontal beam. 

 [Weights were then gradually hung on to the lower ends. Only two were 



found strong enough The one selected stood 450 grams without breaking, 



its diameter being less than 'ooi inch. 



which were brass, and the other two a long watch-spring 

 magnet. The discs were of pith blackened on one side. 



The instrument was floated in a vessel of water, four 

 candles being placed round it to set the arms in rotation. 

 A mark was put on the glass envelope to enable a slight 

 movement to be seen. 



A powerful magnet was now brought near the moving 

 arms, which immediately stopped, and at the same time 

 the glass envelope commenced to revolve in the opposite 

 direction to that in which the arms had been revolving. 

 The movement kept up as long as the candles were 

 burning, and the speed was one revolution in two minutes. 

 On the magnet being removed the arms obeyed the force of 

 radiation from the candles and revolved rapidly, whilst the 

 glass envelope quickly came to rest. The candles were then 

 blown out, and as soon as the whole instrument had come 

 to rest, a bar- magnet was moved alternately from one side 

 to the other of the radiometer, so as to cause the vanes to 

 rotate as if they had been under the influence of a candle. 

 The glass envelope moved about one revolution in three 

 minutes in the same direction as the arms, and on re- 

 versing the direction of movement of arms, the glass 

 envelope changed direction also. This I consider is 

 proof that the internal friction, either of the steel point 

 on the glass socket or the vanes against the residual air, 

 or of both these causes combined, is considerable. Moving 

 the vanes round by the exterior magnet carries the whole 

 envelope round in opposition to the friction of the water 

 against the glass. 



In another communication I propose to give the results 

 of my experiments on the influence of the residual gas on 

 the movement of the radiometer, and also refer to other 

 results which I have recently obtained. 



William Crookes 



ON A NEW ASTRONOMICAL CLOCK^ 



'X*HE object of this communication was to explain to 

 -*- members of the Association and give them an oppor- 

 tunity of seeing in my house in the University a clock 

 which had been described in a communication to the 

 Royal Society, in 1869, entitled "On a New Astronomical 

 Clock and a Pendulum Governor for Uniform Motion." 

 The following description is taken from the Proceedings 

 of the Royal Society for 1869, except a few alterations 

 and additions, and except the drawings, which have not 

 been hitherto published : — 



It seems strange that the dead-beat escapement should 

 still hold its place in the astronomical clock, when its 

 geometrical transformation, the cylinder escapement of 

 the same inventor, Graham, only survives in Geneva 

 watches of the cheaper class. For better portable time- 

 keepers it has been altered, through the vicious rack-and- 

 pinion movement, into the superlatively good detached 

 lever. If it is possible to make astronomical clocks go 

 better than at present by merely giving them a better 

 escapement, it is quite certain that one on the same 

 principle as the detached lever, or as Earnshaw's ship- 

 chronometer escapement, would improve their time- 

 keeping. 



But the irregularities hitherto tolerated in astro- 

 nomical clocks may be due more to the faultiness 

 of the steel and mercury compensation pendulum, with 

 its loosely attached glass jar, and of the mode in 

 which it is hung, and to instaloility of the supporting 

 clock-case or framework, than to imperfection of the 

 escapement and the greatness of the arc of vibration 

 which it requires ; therefore it would be wrong to expect 

 confidently much improvement in the time-keeping merely 

 from improvement of the escapement. I have therefore 

 endeavoured to improve both the compensation for change 



I "On a New Form of Astronomical Clock with Free Pendulum and Inde- 

 pendently Governed Uniform Motion for Escapement Wheel." By Prof. 

 Sir William Thomson, F. R.S, (iJommunicated to Section A of the British 

 Association, Thursday, September 7, 1876.) 



