306 Ritchie on the Equality of 



about a foot long ; the vertical portions, AD, B C, being each 



four or five inches. Near the extremities of the vertical por- 

 tions two small bulbs are blown, for the purpose of holding a 

 part of the coloured liquid and preventing it entering the cylin- 

 drical chambers. The cylindrical chambers may be three or 

 four inches in diameter and half an inch thick, having tin tubes 

 soldered in the rim for receiving the extremities of the glass 

 tube, which is rendered air-tight by common electric cement. 

 The surface of one of the cylindrical chambers is coated with 

 lamp-black, whilst the opposite surface of the other chamber 

 is left perfectly bright. To the glass tube is attached a small 

 scale, divided into any number of equal parts. The liquid 

 which I prefer, on account of the delicacy of its motions, is 

 spirit of wine, tinged in the usual way. 



A cylinder of tin-plate, of the same diameter as the cham- 

 bers of the thermometer and about an inch thick, having one 

 of its sides coated with lamp-black and the other left bright, is 

 to be placed exactly half way between the chambers, and then 

 filled with hot water, when the following phenomena will be 

 observed : 



1. When the coated side of the cylinder D is turned opposite 

 the coated side of the chamber D', the fluid in the stem B C will 

 sink with extreme rapidity. The reason of this is obvious : From 

 the coated side of the cylindrical canister we have an immense 

 quantity of radiant heat shooting out at right angles to the 

 surface, and falling on the powerfully absorbing surface of the 

 cylindrical chamber of the instrument ; whereas, from the 

 opposite metallic surface of the canister we have a very scanty 

 portion of radiant heat, and that, too, falling on a surface 

 which absorbs only a minute portion ; hence the striking diffe- 

 rence between the temperature of the air in the two chambers 

 of the thermometer. 



2. Remove the canister, and again place it exactly in the 



