548 Scientific Proceedings, Royal Dublin Societtj. 



a proper bulk, this force will not be adequate to cause the cylinder 

 to break the surface of the mercury and squeeze the latter down- 

 wards between the edge of the disk and the glass. Thus the 

 mercury column is preserved from falling out of the tube, the free 

 surface at the base being reduced to the small annular space around 

 the top of the cylinder, which annular space may be as narrow as 

 we please so long as contact between disk and tube is avoided. 

 Such a tube may be considerably shaken without the outflow of 

 the mercury occurring, and if some air be let pass up (by inclining 

 the tube) the elasticity of this permits of the column being oscil- 

 lated vertically with considerable violence, without air passing up 

 or the mercury leaking downwards. I may mention here that, in 

 conjunction with Professor Eitzgerald, I have adopted this arrange- 

 ment to the demonstration at lectures of the thermal expansion of 

 air. For if such a tube, with air above the mercury, be jacketed 

 first with ice and then with steam, the sinking of the column will 

 amount to a length of the tube (supposed uniform in bore) which 

 will be represented by 100 units, the length of the column at 0° 

 being 273 such units. Thus a simple demonstration of the abso- 

 lute zero of the air thermometer is provided. To keep the air dry 

 in this arrangement it is sufficient to let a piece of calcium chloride 

 permanently float upon the surface of the mercury. A convenient 

 length for such a tube will be one metre, and the bore one centi- 

 metre. This is visible to a large class. 



Returning to the application of this contrivance to the con- 

 struction of a mercury-glycerine barometer, it is apparent at once 

 that we might have the space in the tube below the disk filled with 

 glycerine, and preserve the open end of the tube dipping below a 

 bath of glycerine. We may thus have a column of glycerine in 

 the tube loaded with a column of mercury. Suppose we make 

 the mercury column 21" in length, and that a vacuum exists above 

 the mercury, the mercury column will stand at such a height in 

 the tube that the pressure at the base of the tube, due to the length 

 of the column of glycerine plus that due to the length of the 

 column of mercury, is equal to that exerted by the atmosphere. 

 Thus, if the mercury barometer stand at 29 inches, then a column 

 of glycerine equivalent to two inches of mercury must stand in the 

 tube. If the barometer rise another tenth inch, such a further height 

 of glycerine as will exert a pressure equal to the tenth of an inch 



