14 DR. F. HORTON OX THE EFFECTS OF CHANGES OF TEMPERATURE 



The fixed mirror was supported on a, small brass stand resting l>y three levelling 

 screws on the floor of the heating jacket (see fig. 1). It could he rotated alxul a 

 vertical axis in order to set the mirror parallel to the one on the vibrator. 



Tin- Mi-tlioil <>f Takiny an Observation, 



In general, on one day, two sets of observations were taken, the first at the 

 temperature of the tap water, and the second at some higher temperature. In order 

 to get the two done in one day it was necessary to leave the water running through 

 the jacket over night, so that in the morning no time should be wasted in waiting for 

 the temperature to become steady. 



A set of observations at the temperature of the tap water having been taken, the 

 heating jacket was emptied and dried by sucking warm dry air through it. It was 

 then connected up to the boiler in the manner already described. As a rule, it would 

 take from 3 to 4 hours for the temperature to become constant, and in the interval 

 the rest of the apparatus was tested and set in order if necessary. The fixed mirror 

 had to be re-set parallel to the moving one at each temperature, for the wire altered 

 its zero position slightly. When the temperature had become constant the plate was 

 set vibrating, the angle through which it moved being observed by placing a candle 

 behind the slit so as to illuminate it continuously. The amplitude of oscillation of 

 the plate was increased until the image of the slit moved over slightly more than 

 22 divisions of the telescope scale. The plate was then left vibrating for a short 

 time in order to get over all the disturbing effects of air currents. The amplitude of 

 swing having damped down to 22 divisions on the eye-piece scale, the time of a 

 chronometer was noticed, and the temperatures of the thermometers read. This was 

 for a determination of the logarithmic decrement of the oscillations which was made 

 with each observation of the period. When the amplitude had further damped down 

 to 20 divisions (=15 minutes of arc) the candle was removed from behind the slit 

 and the observations of the period begun. Since it was times of complete vibrations 

 that were required, a coincidence being taken when the moving mirror was parallel to 

 the fixed one, and moving always in the same direction, an error might have arisen 

 through variations in the zero position of the wire during an experiment. In order 

 to eliminate this, two sets of coincidence periods were taken in which the flashes 

 cither " lost" or " gamed" in opposite directions. 



Everything being ready, and the temperature of the thermometers noted, the 

 observer sits down to the telescope and watches until he sees a coincidence is about 

 to happen. He notices one flash is " early " by a certain amount while the 37th (if 

 37 is the coincidence period) is "late" by another amount. These are noted down 

 He must then go on counting from 1 np to 37 continuously, making a note of each 

 37th. After a time he will observe that a coincidence in the opposite direction is 

 about to happen, i.e., that some flash say the 23rd- -is moving across the field in the 

 opposite direction to the 37th and will soon be coinciding with the fixed flash. This 



