METEOROLOGICAL COMMITTEE. 139 



barometric column, without suffering any change in the 

 height of its surface. 



The cistern is made of well seasoned mahogany. The 

 excavation for containing the mercury, consists of a circular 

 pool of 1.100 inch diameter and of the same depth, for receiv- 

 ing the end of the barometer tube ; tlie upper part of this pool 

 is, however, extended to nearly the diameter of the glass plate 

 before mentioned (3 inches) which covers it, leaving only a 

 narrow rim for supporting the plate and attached tube : this 

 part has a depth of 0.167 inch. The cistern thus propor- 

 tioned, being filled until the mercury shall extend from the 

 tube against the glass plate to a diameter of 2 inches, will 

 admit of fluctuations beyond the barometric range, without 

 the mercury either extending beyond, or leaving the under 

 surface of the plate. This under surface, then, becomes a 

 fixed point from which the measurements for the scale may be 

 accurately laid off. At the coincidence of the lower surface 

 with the tube, a mark was made on the latter; the plate being 

 removed, 30 inches were measured off and another mark 

 made on the tube ; to this mark the scale was laid down. 



The arrangement for the scale consists of a brass tube, 7 

 inches long, and of a diameter to fit over the barometer tube, 

 to which it is firmly cemented. This tube extends two 

 inches above the top of the barometer ; and from the top of 

 the barometer tube, for a length of three inches downwards, 

 is opened on opposite sides one-quarter of its circumference, 

 so as to expose the barometer tube and admit of vision 

 through that part of it which is unoccupied by the mercury. 

 Over this brass tube, a sliding one is fitted of nearly the same 

 length, the lower end of which is brought in coincidence with 

 the top of the mercurial column, when observation is made of 

 its altitude. Attached to the upper end of the tube, is a 

 micrometer screw, which works in the top of the fixed tube ; 

 affording an easy means of bringing the termination of the 

 sliding tube in the same horizontal line with the surface of 

 the mercury. 



On the fixed brass tube a scale is graduated to fiftieths of 

 an inch. The micrometer screw has fifty turns to the inch, 

 and the micrometer head being divided into twenty parts, a 

 subdivision of the scale is effected to thousandths. 



