454 Convection of Light in Moving Gases. 



itself " boiled " very badly, except in the cool mornings. 

 The heat of the sun during the day caused variations in the 

 temperature of the air inside the pipes, and the convection 

 currents deflected the light in all sorts of ways. This was 

 to some extent avoided by covering the entire length of the- 

 pipes b} T bamboo screens, but the most satisfactory results 

 were obtained only in the mornings or an hour or two after 

 the sun had gone down. 



To test whether or not the paths traversed by the two 

 interfering beams were identical, the method suggested by 

 MicheJson was employed. A plane-parallel plate of glass 

 was inserted in front of one of the ends of the tube, and the 

 effect of its rotation on the fringes was observed in the 

 microscope. With the final adjustments no displacement of 

 the fringes could be observed by this means, and there seemed 

 to be no reason to doubt that the paths were really identical 

 and not merely parallel. 



The current of air was now turned on, but it was at once 

 apparent that the air was not flowing at a constant rate, but 

 was being driven through the tubes in puffs : and although 

 nothing happened to the fringes which, whenever visible, 

 appeared in the same position as when no current of air was 

 flowing, yet the spots of light themselves "boiled" very 

 badly indeed. An attempt was made to overcome this 

 difficulty by inserting a reservoir of air to steady the motion, 

 and, after some preliminary trials with a brickwork and 

 cement reservoir which developed a leak and proved unsatis- 

 factory, a large wooden box, 4 ft. X 4 ft. x 6 ft., covered over 

 with galvanized-iron sheets was installed, and inserted in the 

 line of flow of the air between the pipe-line and the blower 

 (see fig. 3, PI. VI.). This successfully checked the oscil- 

 lations in the speed of the current of air, and the spot of 

 light in the microscope containing the interference fringes 

 co aid be steadily seen, even with the current of air running. 



To measure the velocity of the air-current, a pair of Pitot 

 tubes with water-manometer were inserted in the channel 

 between the pipe-line and steadying reservoir. 



3. Results. 



When the arrangements including the steadying reservoir 

 were complete in April 1921, it was found that the engine 

 and blower were unequal to the task of drawing the air 

 through the system at the originally estimated velocity of 

 50 metres per second, and that a speed of only about 

 20 metres per second could be attained. The hot weather 



