on the terrestrial Rays that occasion Heat. 529 



board or vane; one towards the right, the other towards the 

 left ; but so as to meet in the middle, and apparently to make 

 but one when placed side by side. The vanes are covered with 

 a piece of fair white paper, which is to reflect, or rather to scatter 

 light in every direction. To one end of the board is fixed a circular 

 piece of wood, with an opening in it, which is afterwards to be 

 shut up by a small moveable piece,* intended for placing the 

 transmitting objects upon. This moveable piece contains two 

 holes, at the distance of \\ inch from centre to centre, and 

 £ inch in diameter each. Against the circular wooden screen, 

 and close over the opening in it, is placed a lantern containing 

 a lamp.-f Its construction is such as to admit a current of air 

 to feed the flame from below, by means of a false bottom, and 

 to let it out by a covered roof; and the whole of the light, by 

 the usual contrivance of dark lanterns, is thus kept within, so 

 as to leave the room in perfect darkness. In the front, that is 

 towards the vanes, the lantern has a sliding door of tin-plate, 

 in which there is a parallelogram mic hole, covered with a spout 

 five inches long, of the same shape. Two or three such doors, 

 with different spouts and openings, will be required to be put 

 in, according to the experiments to be made; but the first will 

 do for most of them. 



A narrow arm is fastened to the long board, which advances 

 about three feet beyond the screen, and carries a circular piece 

 of pasteboard, that has an adjustable hole in the centre, through 

 which the observer is to look when the experiments are to be 

 made. At the farther end of the long board is a pulley, over 

 which a string, fastened to the back of the slider that carries 

 one of the vanes, is made to pass. This string returns under the 



* See Plate XXVI. Fig. 4. $ See Fig. 5. 



