45, CORNHILL, B.C., AND 122, KEGENT STREET, W., LONDON. 



19 



SELF-REGISTERING BAROMETERS. 



For many years a good and accurate self-recording barometer was much 

 desired. This want is now satisfactorily supplied, not by one, but by several 

 descriptions of apparatus. The first was the design of Admiral Sir A. Milne, 

 who himself constructed, in 1857, we believe, the original instrument, which 

 he used with much success. 



25. Negretti and Zamtora's improved Self- 

 Registering Mercurial Barometer or Baro- 

 graph. In this instrument the various parts of 

 the mechanism have been so modified and arranged 

 that the record on the papers is obtained with the 

 greatest precision and delicacy. The engraving 

 (fig. 21) will give the general details. It should, 

 however, be mentioned, that it is not a picture of 

 the outward appearance of the instrument. The 

 position of the barometer should be behind the 

 clock ; it is represented on one side merely for the 

 purpose of clearly illustrating the arrangement. 

 The instrument has a large syphon barometer tube, 

 in which the mercurial column is represented. 

 On the mercury at A, floats a glass weight, 

 attached to a silk cord, the other end of which is 

 connected to the top of the arched head on the 

 short arm of a lever-beam. The long arm of this 

 beam is twice the length of the short arm, for the 

 following reason. As the mercury falls in the long 

 limb, it rises through an equal space in the short 

 limb of .the tube, and vice versa. But the barometric 

 column is the difference of height of the mercury 

 in the two limbs ; hence the rise or fall of the float 

 through half-an-inch will correspond to a decrease 

 FIG. 21. or an increase of the barometric column of one 



inch. In order, then, to record truly the movements of the mercurial column, 

 and not those of the float, the arm of the beam connected with the float is only 

 half the radius of the other arm. From the top of the large arched head a piece 

 of watch-chain descends, and is attached to the marker, B, which properly 

 counterpoises the float, A, and is capable of easy movement along a groove in 

 a brass bar, so as to indicate the barometric height on an ivory scale, C, 

 fixed on the same vertical framing. On the opposite side of the marker, J?, 

 is a metallic point, which faces the registration sheet and is nearly in 



