346 MB BLACKADDER on Meteorological Instruments 



phere and the mercury of the barometer, than which certainly, 

 nothing can be more simple. If, at a given instant, the com- 

 munication between the air and the mercury be cut off, the 

 height of the mercurial column must remain unaltered by any 

 change in the pressure of the atmosphere, until such time as the 

 communication is restored. And nothing is more simple than 

 the means by which this interruption of communication can be 

 effected, during absence, and at any given instant. 



An accident, by which my instrument was broken, has put it 

 out of my power to exhibit a barometer constructed on this 

 principle. The principle, however, is so self-evident, and the 

 method of putting it into execution so simple, that little or no 

 illustration seems requisite. I have therefore made a section of 

 the cistern *, which will be quite sufficient for any explanation 

 that may be considered necessary. The cistern is made of iron, 

 and is about two inches in diameter the depth of mercury in 

 the cistern, and the distance between the surface of the mercury 

 and the top of the cistern must be as small as the correct opera- 

 tion of the instrument will admit of. To the air-orifice is at- 

 tached an air-tight stop-cock, and by means of a lever, or other 

 piece of mechanism, connected with a timepiece, the stop-cock 

 may be shut at any given instant. In this way he can ascertain 

 the exact height of the mercurial column during absence, and at 

 any hour or minute that may be fixed upon. 



As we found to be easily effected in the case of the Thermo- 

 meter and hygrometer, so also in this, by combining several such 

 instruments in one piece of mechanism, we can have the exact 

 height of the barometer every hour in the course of the day and 

 night. Thus f, seven barometers, arranged at equal distances 

 around a hollow column of wood, four inches in diameter, and 



* See Fig. 1. Plate XIII. f See Fig. 2. Plate XIII. 



