392 Of the Propagation of Heat 



diately by means of the Heat absorbed by or generated 

 in the sides of the tumbler. This experiment appears 

 to me to be important in many respects ; but it would 

 be foreign to my present purpose to engage in an in- 

 vestigation of the subject with which it is most inti- 

 mately connected. 



I cannot finish this Essay without giving my reader 

 an account of one more experiment, the result of which 

 was not only quite unexpected, but uncommonly inter- 

 esting. 



Happening accidentally to place in a window the 

 little instrument I had contrived for rendering visible 

 the internal motions which are occasioned in water when 

 Heat is propagated in that fluid,* as it was winter, and 

 the room was warmed by a German stove, that side of 

 the instrument which happened to be nearest the window 

 being exposed to a current of cold air, while the instru- 

 ment received Heat continually on the other side from 

 the warmer air of the room, the liquid in the instrument 

 was thrown into motions which never ceased, and af- 

 forded a very interesting sight. 



With a view merely to amuse myself, and the friends 

 who should happen to call in to visit me, and without 

 the smallest expectation of making any new discoveries, 

 I contrived, and caused to be executed, the instrument I 

 am now about to describe, which I thought could not fail 

 to render these motions perpetual, and exhibit them in a 

 striking manner. 



A flat box was formed of two equal panes, each 13 

 inches high, and 10^ inches wide, of fine ground glass, 

 fitted into a square frame of brass in such a manner that 

 these two panes (which are parallel to each other) are at 



* For a description of this instrument see Chapter II. of this Essay. 



