148 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



with mercury, and in this state is inverted, and the end 

 plunged in a jar of the same fluid ; to the top of the 

 ball is attached a string, which, after passing over the 

 pulleys, D and E, is finally fastened to the window. F. 

 When the heat of the apartment rarities the air in the ball, 

 the mercury is driven out, and the ball and tube being 

 thereby lightened, rise and suffer the window to swing itself 

 open. An increase of cold will produce the contrary effect. 



Price, $25.00. 



