II. MINING. 733 



This instrument is specially for the use of mining engineers and surveyors, 

 for the purpose of readily ascertaining slight variations in gradients, levels, &c., 

 and, from its extreme sensitiveness, will be found of considerable utility in 

 mining and surveying work generally. Besides its extreme sensitiveness, the 

 speciality claimed for the instrument is an arrangement of the scale of altitudes 

 which admits of subdivision by a vernier, hitherto impracticable, owing to the 

 altitude scale in ordinary use being a gradually diminishing one, to which a 

 vernier cannot be applied. In the present instrument the action has been so 

 adjusted as to give accurate readings upon a regular scale of altitudes, the 

 barometrical scale of inches being made progressive in length, so as to afford 

 the correct relative readings with the scale of altitudes. For mining opera- 

 tions the entire circle of the dial is graduated to represent 6 in. of the mer- 

 curial column that is, from 27" to 33". This scale affords observations 

 from about 2,000 ft. below sea level to 4,000 ft. above. The finest division 

 of the altitude scale (l-100th) represents 10 feet measurement, which can be 

 again divided by the vernier scale to single feet. The vernier scale is moved 

 by rack- work adjustment, and a lens, which rotates on the outer circumference 

 of the instrument, facilitates the reading of minute quantities. For surface 

 surveying purposes, where it is not required to be used below sea level, the 

 instrument is made with the scale divided from 25 to 31 in., thus giving an 

 altitude scale of 6,000 ft. above the sea level only ; and with this open scale, 

 and the assistance of the vernier, the same minute readings to single feet may 

 easily be taken. The instruments are also constructed for measuring much 

 greater altitudes, that is 10,000, 15,000, or 20,000 ft., but with these scales the 

 measurement cannot be made quite so minute as in the more open scales. 

 The instrument is 4^ inches in diameter, and is provided with a leather sling 

 case, thus making it sufficiently portable for all practicable purposes. 



3386b. Davis's Improved Colliery Barometer, specially 

 adapted for moist climates. John Davis and Son. 



The necessity of a travelling screw is dispensed with by choking partially the 

 neck of the tube, thus preventing the possibility of the mercury breaking the 

 end of the tube by violent concussion in transit. The travelling screw (or the 

 screw at bottom for driving the mercury to the top of the tube) being dispensed 

 with, the cistern can be made entirely of boxwood. All the parts of the case 

 are well screwed together, thus making the instrument suitable for moist 

 atmospheres, such as pit banks. 



3387. Air Meter, used for the ventilation of mines and other 

 large buildings. Francis Pastorelli. 



This consists of a horizontal box, with a dial, upon which are divided circles 

 and indexes ; this is mounted upon three vertical pillars, fixed into a brass 

 base ; solidly attached is a vertical ring ; within this are eight vanes fixed to 

 one end of a horizontal axis, the other terminating with an endless screw, 

 which works a series of wheels in the box, and their revolutions are recorded 

 by indexes on the dial ; some of the working parts are jewelled to obtain 

 the minimum amount of friction. 



The large circle is divided into 100 parts which represent feet; one revolu- 

 tion of its hand is equal to 100 feet; now as the hand of the small circle 

 goes 1 times as slow, it is evident, that, while the former makes one complete 

 revolution, the latter will only have made one-tenth of a revolution ; therefore, 

 when it has made one complete revolution, it will indicate 1,000 feet. The 

 small circles are lettered hundreds, thousands, &c., up to 10 millions, and 

 numbered 1, 2, 3, 4, 5, 6, 7, 8, 9, 0. 



For measuring an air current of a low velocity the readings on the large 

 circle are taken, for a high velocity on both. 



