1856] WRITINGS OF JOSEPH HENRY. 421 



The ceiling is twenty-five feet high, and therefore within 

 the limit of perceptibility. It is perfectly smooth and un- 

 broken with the exception of an oval opening nearly over 

 the speaker's platform through which light is admitted. 



No echo is given off from the ceiling, while this assists the 

 hearing in the gallery by the reflection to that place of 

 the oblique rays. 



The architecture of this room is due to Captain B. S. 

 Alexander, of the corps of Topographical Engineers. He 

 fully appreciated all the principles of sound which I have 

 given, and varied his plans until all the required conditions 

 as far as possible were fulfilled. 



ACCOUNT OF A LARGE SULPHURIC-ACID BAROMETER IN THE 

 HALL OF THE SMITHSONIAN INSTITUTION. 



(Proceedings American Association Adv. of Science, vol. x, pp. 13.5-138.) 



August 23, 1856. 



The opinion has been frequently advanced that a barom- 

 eter in wjiich the material used to balance the pressure of 

 the atmosphere is of less specific gravity than mercury, and 

 consequently of a wider range of fluctuation, might throw 

 some new light on several important points of meteorology. 

 The fluid usually proposed for this purpose has been oil or 

 water, the viscid character of the former and its tendency 

 to a change of condition has induced a preference for the 

 latter. Several water-barometers have accordingly been con- 

 structed ; but as far as I am informed, the indications of the 

 instruments have not been reliable. 



Mariotte used one of this character ; also Otto von Guericke 

 constructed a philosophical toy to which he gave the name 

 of aeroscope, on the principle of a water-barometer. It 

 consisted of a tube more than thirty feet high elevated on a 

 long wall and terminated by a tall and rather wide glass 

 cylinder hermetically sealed, in which was placed a toy in 

 the shape of a man. All the tube except a portion of 



