574 SEC. 14. METEOROLOGY. 



metres. Each degree can easily be further subdivided by estimation into ten 

 parts. One such tenth corresponds to about ^ mm. of the mercurial column, 

 or a difference in height of meter. 



The instrument is chiefly destined for the preparatory tracings of railway 

 lines and similar engineering works, and also for the observation of minute 

 oscillations in the atmospheric pressure. Its construction is considerably 

 simpler than usual ; optical means serve to magnify the movements of the 

 vacuum box, instead of the customary transmission by lever ; the instrument 

 is easily handled in the field. Care has been taken in the correct application 

 of the thermometer, which is to indicate the temperature of the most impor- 

 tant parts of the instrument. 



282Oa. Field's Aneroid. L. Casella. 



2820b. Open Range Aneroid, with registering indices. 



L. Casella. 

 282Oc. Gold Band Aneroid. L. Casella. 



282Od. Gary's Improved hardy Aneroid Barometer 



made strong expressly for the use of travellers, in improved wood 

 and leather case and sling. Henry Porter. 



c. SYMPIESOMETERS. 



2821. Sympiesometer, a sensitive instrument for sea use. 



Francis PastorelU. 



This consists of a syphon tube, containing a volume of air and a fixed fluid 

 that partly fills the tube, also a thermometer. Its principle of action is upon 

 Mariotte or Boyle's law. By an increase or decrease of the weight of the 

 atmosphere the fluid is raised or lowered (arising from the elasticity of the 

 enclosed air), through equal distances for each barometric inch if the con- 

 fined air were unaffected by varying heats, but as it is affected by temperature 

 this error is allowed for by a temperature scale. To take a reading note first 

 the temperature of the thermometer ; now set the pointer attached to the 

 movable scale of inches, by means of its rack motion, to the corresponding 

 degrees of temperature of the syphon tube, the position of the fluid indicates 

 the height of the barometer. This instrument is considered by many a most 

 valuable instrument ; it is more sensitive than the barometer, and when 

 accurately constructed should give good results. 



III. SPECIAL THERMOMETERS. 



2823. Old Floating Thermometer, by Gay Lussac and 

 Collardeau, of Paris. G. J. Symons. 



2824. Thermometer of Translation or Integrator of varia- 

 tions of temperature. Scottish Meteorological Society. 



The bar of zinc is fixed at its lower end during expansion by the needle 

 points catching in the teeth of the rack below, so as to produce lengthening 

 upwards, while during contraction the bar is held by the needle at the top, so 

 that the shrinking is upwards. In this way the centre of gravity is moved 

 upwards. The total annual march or creep of the bar will measure the total 

 amount of fluctuation of temperature. Designed by Thomas Stevenson, C.E., 

 Honorary Secretary. 



