134 NEGEETTI AND ZAMBEA, HOLBOEN YIADUCT, E.G. 



DIAL AND PEDIMENT BAROMETERS, 



THE ordinary household Barometers or Weather Glasses are constructed in 

 two forms, viz., the Wheel or Dial Barometer, and the Pediment or Upright 

 Barometer. The former reading by an extended circular scale, and the latter 

 from the actual mercurial column. 



The Dial Barometer is mounted with the syphon form of tube as shown 

 in our Diagram, the shorter -limb of the syphon being about six or eight inches 

 long. This obviates the use of a cistern, for with sufficient mercury in the 

 short tube, that in the longer one will be balanced at a varying height in 

 accordance with the increased or diminished pressure of the atmosphere. 



This form of Barometer was first constructed by the celebrated philosopher 

 Dr. Hook, in 1667 ; the principal advantage of the dial arrangement is that by 

 it a small movement in the mercurial column is magnified and made very 

 apparent, a tenth of an inch rise or fall being represented by the index moving 

 over nearly one inch on the dial ; this enables the unscientifiVobserver to notice 

 quickly if the Barometer be rising or falling and estimate the probability of 

 fine or wet weather. 



On the top of the mercury in the short limb of the tube is suspended a 

 glass float, by a silk cord which passes two or three times round a small brass 

 wheel or pulley ; at the other end of this silk cord is placed a counterpoising 

 glass weight, moving freely in a second tube placed at the side of the syphon 

 for the purpose of steadying the weight. 



The axis of the pulley is carried through the wood frame of the Barometer 

 to the front of the instrument, where the movement of the mercury is shown 

 by a light index hand attached to it, traversing a divided dial. 



As the mercury in the Barometer tube rises, the silk cord descends, 

 causing the index hand to move to the right ; on the contrary, as the mercury 

 falls the index will be carried in the reverse direction to the left. 



The graduations on the dial represent the actual inches of a Standard 

 Barometer Scale, extended as previously described, giving what is termed a 

 very open scale. 



If the mercury rises half an inch in the long tube it will fall half an inch 

 in the shorter one. Therefore, as the mercury rises half an inch in the one 

 tube and falls half an inch in the other, the length of the barometrical column 

 has increased one inch, but the movement has only been through half an inch 

 this amount of movement transferred to the pulley causes the index hand to 

 indicate an inch movement on the dial, say from twenty-nine to thirty inches. 



