24 7 
VELOCITY OF WHITE AHD OF OOLOUEED LIGHT. 
and observed in different positions of the fly-wheel. Thus we have a rigid mass 
rotating about a vertical axis with no cause for irregular motion except the acceleration 
produced by the weight and a slight friction, which combine to produce an accelerating 
or retarding force which may be regarded as constant. Two cylinders, exactly similar, 
were employed, and these could be interchanged. The brass cylinders were covered 
with a thin layer of smoke from a paraffin lamp. 
The marks made by the currents of electricity from the clock and wheel respectively, 
were produced by two aluminium points attached to two springs, which were 
attracted by the poles of two electro-magnets in the circuits of the clock and wheel 
respectively. Thus, while the pens are in contact with the cylinder if there be no 
currents passing, and the fly-wheel be rotated, two circles are traced upon the cylinder. 
The electro-magnets are mounted on a stand which slides up and down one of the 
vertical brass pillars by means of a rack and pinion. Thus the assistant can, by 
turning the pinion, convert the two circular marks into spirals. We judged that it 
was much better to make an assistant perform this work than to use up the energy of 
the fly-wheel in the same way, as is often done in chronographs. Moreover, he turns 
the pinion only while an observation is being recorded, and not in every case when 
the fly-wheel is rotating, thus effecting a great saving in the space on the cylinder. 
The electro-magnets are not fixed rigidly to this support which moves up and down 
on the vertical column. They are attached to it by a vertical axis about which they 
have a small motion. Thus we are able to keep the pens in contact with the cylinder, 
or not, as we please. Ordinarily the pens are kept away from the cylinder by a light 
spring. By sending a current of electricity through a third electro-magnet, the pens 
are brought into contact with the cylinder. This current is sent by the observer by 
means of a contact maker, a few seconds before making an observation. When the 
assistant sees this he moves the pinion so as to cause the pens to describe a spiral, and 
the observer, at the instant of an observation, breaks and makes contact rapidly. 
This leaves a break on the smoked cylinder for about one-eighth of a second, the 
beginning of the break indicating the time of the observation. 
When it is desired to read off the records of observations the cylinder is first taken 
off, and the collar which connected it rigidly to the fly-wheel removed. A ring, to 
which an arm carrying a vernier is attached, is now screwed to the centre of the upper 
triangle, and forms a bearing in which the axis of the fly-wheel can work. This part 
of the axis is conical, having the smallest diameter above. On this conical pivot 
a conical collar, which carries a 5-inch divided circle, is jammed. The cylinder is 
replaced with the lower end of its axis resting on the pointed upper end of the axis of 
the fly-wheel. It is prevented from rotating independently of the fly-wheel by an 
upright piece of metal and an upright spring, which are attached to the divided circle, 
and which catch between them one of four radial arms at the base of the cylinder. 
It has been stated that one of the vertical pillars carries the marking pens or 
styles. The other pillar bears an arm to which is attached a microscope with cross 
