182 Detection of Heat by Convection. 



graduated scale was placed under the needle to measure 

 the motion produced. 



With this arrangement some very encouraging results 

 have been obtained. The following examples will serve to 

 illustrate : The end of an iron wire after an almost mo- 

 mentary pressure between the fingers, held near the needle, 

 quickly drew it out of place. A hammer falling by its 

 own weight through a distance of 15 inches and striking 

 upon the end of the wire, evolved so much heat that the 

 needle was wafted through an arc of 60°. A small glass 

 tube gently rapped upon the edge of the table and then 

 placed inside the instrument gave motion to the needle, 

 distinctly seen by the most distant member of a large class. 



JN"ext to the expensive Thermo-pile this Convection Ther- 

 moscope is probably the most delicate indicator of heat at 

 present known. 



