26 



NEGRETTI AND ZAMBRA, HOLBOKN VIADUCT, E.G., 



FIG. 27. 



Our fig. 27 shows one of the most convenient arrangements yet introduced, 

 viz., a Watch-sized Aneroid, with a reliable Thermometer and Compass. The 

 hinged leather case containing the three instruments, being but little larger 

 than an ordinary portemonnaie. Price, see page 29. 



31. Measurement of Heights by the Aneroid. The dial of the Watch 

 Aneroid for determining altitudes is engraved with two scales in concentric 

 circles, the inner circle being divided into inches and tenths of an inch, corre- 

 sponding with the scale of the Mercurial column of a Standard Barometer. 

 The outer circle is divided into spaces representing 100 feet, each tenth division 

 being numbered as 1,000, 2,000, &c. The zero point of this circle corresponds 

 with 31 inches of the Barometer scale, for this reason, that the Barometer never 

 rises so high as 31 inches, consequently, our scale of feet is always outside the 

 weather range. The zero of the feet scale has nothing whatever to do with the 

 sea-level, that is a variable point and must be determined at the time of 

 observation either by actual measurement at half tide level, or by computation 

 from a known height. 



32. Measurement of Altitudes above Sea Level. In order to deter- 

 mine the height of any station above the sea-level with this instrument, we must 

 notice at what point it stands at the shore ; we then ascend, and on reaching 

 the desired point, observe the position of the index on the dial. We then 

 deduct the number of feet opposite the reading on starting from that against 

 the reading at the elevated station, this gives the height above the level of the 

 sea. Thus, if at sea-level, the barometer stands at 30 inches, and at the elevation 

 it stands at 26 inches we get 900 feet, deducted from 4,800 feet, giving us a 

 height of 3,900 feet, and so on for the other points of the scale. 



When great accuracy is required, simultaneous observations must be taken 

 at the two stations to obviate any error that might arise from a change of 

 weather between the times of observation. 



Further instructions for altitude measurement will be found in Negretti 

 and Zambra's Treatise on Meteorological Instruments. 



