STANDARD 



METEOROLOGICAL INSTRUMENTS, 



MANUFACTURED BY 



L. CASELLA. 



BAKOMETEES. 



1. Standard Barometer (figs. 1 and 1*, p. 3). The construction of this barometer is that 



known as Fortin's, in which the mercury in the cistern is adjusted, at each 

 observation, to a fixed ivory point, which is the zero of the scale. The 

 mercury is boiled in the tube, which is 0*45 inch, internal diameter. The 

 cistern is made partly of glass, to admit of the zero of the scale being visible, 

 and the mercury is adjustable to the zero, or ivory point, by means of a 

 thumb-screw acting upon a flexible base. The vernier reads to l-500th part 

 of an inch, or, by estimation, to "001 inch, and is adjusted by a rack and 

 pinion motion. In front of the barometer a thermometer is attached, in 

 contact with the tube, with divisions etched on the stem. For facility of 

 reading, a sheet of white note paper should be placed behind the scale. The 

 barometer is mounted in a brass frame, and suspended from a bracket 

 at the top of a mahogany board, so as to ensure perpendicularity. At the 

 bottom of the board is a socket, with clamping screws for steadying the baro- 

 meter in a vertical position, when an observation is made. The instrument 

 is so mounted that it can be turned at pleasure to any source of light 



10 



2. STANDARD BAROMETER, precisely as above, but not so highly finished, inside 



diameter of tube, 0'4D inch. . . . . . 8 10 



(If with Millimetre Scale additional, 100 extra.) 



3. Standard Barometer of extra large size, tube 07 inch, internal diameter, 



with a thermometer immersed in a tube of mercury (at the side) of the 

 same diameter as the barometer tube ; specially suited for public observatories 



22 



Barometers on Fortin's principle are the most reliable. The index error can suffer no 

 change from lapse of time, because it is independent of the loss of mercury from oxidation, 

 etc. ; and, should any air find access into the tube, it can be easily known, and readily re- 

 moved. To know if air has entered the tube, take down the barometer and incline it 

 gently till the mercury reaches the top, when, if air be present, a soft dull tap will be heard ; 

 but if there is no air present, then a sharp clear click will be elicited. To remove air from 

 the tube, incline the instrument gently as above, and invert it so as to allow the air to pass 

 slowly into the cistern. If the quantity be very small, the head may be tapped slightly on 

 the ground to facilitate its exit. It is best, however, to prevent the admission of air whenever 

 possible. 



Instructions. When sent into the country or abroad, the barometer is packed apart from the 

 mahogany board, in some soft elastic material, the mercury being screwed up so as to fill the 

 tube and cistern. It should be unpacked carefully, but not handled until a position has been 

 selected for it. The barometer may be placed in any convenient room, where it is not near a 

 fire or exposed to the sun's rays. It should be in a good light, with the scale about five 

 feet from the ground, so that the zero point in the cistern, and the vernier on the scale may 



B 



