METEOROLOGICAL INSTRUMENTS. 125 



cury in the glass cistern to zero, by just shutting out the 

 light from passing between it and the upper edges of the 

 above-named slits ; and secondly, by screwing it up, it forces 

 the quicksilver upwards, and by filling every part of the 

 tube, renders the instrument portable. 



The divided scale on the upper part, is subdivided, by 

 the help of a vernier, to the one thousandth of an inch. 

 The screw, c, at the top, moves a sliding piece, on which 

 the vernier scale is divided, the zero of which is at the 

 lower end of the piece. In taking the height of the mer- 

 cury, this sliding piece is brought down and set nearly by 

 the hand, and the contact of the zero of the vernier with 

 the top of the mercurial column is then perfected by the 

 screw, c, which moves the vernier the small quantity that 

 may be required, just to exclude the light from passing 

 between the lower edges of the sliding piece, and the 

 spherical surface of the mercury. 



The barometer is attached to the stand by a ring, in 

 which it turns ronnd with a smooth and steady motion, for the 

 purpose of placing it in the best light for reading off, &c. ; 

 and the tripod stand, when closed, forms a safe and con- 

 venient packing case for the instrument. 



A thermometer is always attached to the lower part of 

 the barometer, to indicate its temperature, while another, 

 detached from the instrument, is employed at the same 

 time, to show the temperature of the surrounding air. 



The barometrical method of determining differences of 

 level, is founded upon the principle that the strata of air 

 decrease in density, in a geometrical proportion, when the 

 elevations above the surface of the earth increase in an 

 arithmetical one. Therefore, from the known relation be- 

 tween the densities and the elevations, we can discover the 

 elevations by observations made on the densities by means 

 of the barometer. Price, $50.00. 



The Wheel Barometer. (Fig. 612, next page.) The 

 wheel barometer consists of a glass tube, closed at one end, 

 and the other open, and bent upwards, and of thirty-two 

 inches or upwards in length exclusive of the bend ; the tube 

 is entirely filled with prepared mercury, which, on placing 

 in a vertical position, will subside in the closed end of the 

 tube and rise in the open tube that is bent upwards, until 

 the difference in the levels will be equal to the height of a 

 11* 



