192 LECTURES TO SCIENCE TEACHERS. 



reflections from the sides ; consequently when aqueous 

 vapour was employed it was deposited on the sides of the 

 tube and diminished the radiation. It seems that this is the 

 explanation of the very large diminution in the amount of 

 heat transmitted when aqueous vapour was employed in 

 this tube. The celebrated Dr. Magnus employed a dif- 

 ferent form of apparatus, and also repeated this experiment, 

 and he came to the conclusion that aqueous vapour is not so 

 enormously absorptive of the ultra red rays as Professor 

 Tyndall found ; but in other cases Dr. Magnus's results 

 were in complete accordance with Professor Tyndall's. 



In conclusion I would draw attention to the apparatus 

 of Professor Balfour Stewart for determining the radiation 

 of the sun at any one observatory day after day. It consists 

 simply of an iron case which has a motion in azimuth and 

 altitude, into which a thermometer can be screwed. This 

 pointed directly towards the sun, and the sun's heat is 

 concentrated, by means of a lens, upon the bulb in the 

 interior of the iron case. I have not time to explain the 

 principle of it, but in all probability this method will give 

 the most constant and most clear method ever devised for 

 measuring the amount of radiant heat from the sun at 

 different times. This is a problem which has always 

 offered the greatest difficulty, and a great deal of credit is 

 due to Professor Balfour Stewart for this invention, which 

 will probably lead to more accurate measurements than 

 have yet been obtained. The subject of radiant heat is so 

 extensive that it would have been interesting to have 

 devoted several lectures to the subject, but I have at- 

 tempted to-day to confine myself as far as possible to that 

 part which connects the theory of heat with the theory of 

 light. 



