348 



Prof. W. 0. Roberts-Austen. 



[Mar. 12, 



zontal slit, AB, behind which a sensitised photographic plate, C, 

 is drawn vertically, past the slit, by means of gearing, D, 

 driven by clockwork. The ray from the fixed mirror is interrupted 

 periodically by the vane, E, and a beaded datum line is given which 

 enables any irregularity in the advance of the plate to be detected. 



The amount of divergence from its datum line of the spot of light 

 reflected by the movable mirror at any given moment bears a rela- 

 tion (which can readily be found by calibration) to the temperature to 

 which the thermo-jnnction X is heated, and variations of tempera- 

 ture are recorded by a curve which is the resultant of the upward 

 movement of the plate and the horizontal movement of the spot of 

 light. The complete arrangement is shown in the diagram fig. 1. 



FIG. 1. 



The portion of the arrangement in which the thermo-junction is 

 placed is also shown in fig. 2, which is drawn on a larger scale than 

 fig. 1, the same letters being used in both. 



The thermo- junction X is inserted in a tubulure, T, of a specially 

 constructed crucible of plumbago, c, which contains about 5 oz. of 

 pure molten gold, and is allowed to cool down slowly inside a vessel, 



a, of silver 105 mm. diameter, and polished internally. The cylinder, 



b, is of tin plate, polished internally and blackened outside. 



