IT. BAROMETERS. 685 



2818. Two Metal Barometers, exhibitor's construction, with 

 visible movement. /?. Deutschbein, Hamburg. 



2819. Two Spring Barometers, exhibitor's construction, 

 with visible movement. R. Deutschbein, Hamburg. 



The first four are house barometers, distinguished by good workmanship, 

 shape, and cheapness. The last two instruments are spring barometers, 

 specially adapted to meteorological observations, determination of heights, &c. 



The aneroid barometers of the Reitz system are metal barometers with a 

 vacuum box according to Vidi. The movements of the box are read off by 

 means of a microscope on a scale which is divided into hundredths of milli- 

 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 applica- 

 tion of the thermometer, which is to indicate the temperature of the most 

 important parts of the instrument. 



2820a. Field's Engineering Aneroid, with extra com- 

 pensation for the temperature of the air. L. Casella. 



2820b. Open Range Aneroid, with registering indices, most 

 portable and convenient for showing the maximum and minimum 

 pressure of the air. L. Casella. 



282Oc. Aneroid with Gold Band, for moist climates and 

 coal mines. 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. 



282Oe. Wheel Aneroid Barometer on stand. 



G. Washington Moon. 



c. SYMPIESOMETERS. 



2821. Sympiesometer, a sensitive instrument for sea use. 



Francis Pastorelli. 



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 au 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 



