JoLY — On a Mercury-Glycerine Barometer. 551 



that the cylinder was made in the form of a hollow ivory box into 

 which, by inserting little tinfoil weights, the upward pressure was 

 regulated. It is important that the upper edge of the cylinder be 

 sharp and rectangular, not rounded, as will surely be the case if 

 glass paper be used in finishing it. An alternative and better form 

 of the float is shown in fig. B. There will be less drag upon this 

 in its motion through the glycerine. The cylinder is reduced to 

 the dimensions of a disk. It is kept from jamming in the tube by 

 the four guiding pins projecting from the float, or these pins 

 may be placed below the disk attached to a downward pro- 

 longation of the shank of the float. By shaping a conical cavity 

 in the bottom of the disk or cylinder a chance air bubble rising 

 through the glycerine will be retained beneath the float, where, 

 except it be of considerable size, it can do no harm. 



I have found that distilled water, boiled and cooled in a closed 

 vessel to deprive it of air, serves in place of glycerine except very 

 accurate readings are required. A correction for the vapour tension 

 is of course applied. I have thought of floating a small thermometer 

 upon the surface of the mercury within the tube. The readings of 

 this would be clearly visible through the walls of the tube, and 

 would afford accurately the temperature of the vapour above the 

 mercury, and of the mercury and water, enabling corrections to be 

 applied. In the case of water I have not as yet detected any creep- 

 ing of the water past the column of mercury. 



The use of creosote has also suggested itself. It is at present 

 under trial. It appears to possess all the requisite qualities. Low 

 vapour tension ; specific gravity 1*07', about ; and does not adhere 

 to the glass as does glycerine, the latter property being that most 

 objectionable in the case of glycerine. Again creosote appears 

 unaffected by exposure to the atmosphere, and has no corrosive 

 properties rendering it difficult to handle. It is a clear liquid, 

 colourless or yellowish, possessing an odour of tar. 



