45, CORNHILL, E.G., AND 122, "REGENT STREET, W., LONDON. 



Fortin's barometer cistern shown in section by fig. 3, is 

 formed of a glass cylinder, which allows of the level of the 

 mercury within being seen. The bottom of the cylinder is 

 made of flexible leather, like a bag, so as to allow of being 

 pushed up or lowered by means of a screw, D B, worked from 

 beneath. This screw moves through the bottom of a brass 

 cylinder, C C, which is fixed outside, and protects the glass 

 cylinder containing the mercury. At the top of the interior 

 of the cistern is fixed a small piece of ivory, .A, the point of 

 which exactly coincides with the zero of the scale. This 



screw and moveable cistern-bottom FIG. 3. 



serve also to render the barometer portable, by con- 



fining the mercury in the tube, and preventing its 



descending into the cistern. 



Fig. 4 exhibits the external construction of the 

 cistern portion of a standard barometer. S S are 

 metal screws that secure the glass cylinder or cistern 

 G Gr partly filled with mercury, M, through this the 

 tube T passes down into the flexible leather bag, with 

 which the instrument is adjusted or made portable 

 by the screw, D B, as previously described. At P is 

 shown the white ivory zero point to which the level of 

 the mercury in the glass cistern is always corrected 

 previous to reading off the height of the mercurial 

 column. This ivory point is seen at A in the section 

 FIG. 4. fi o- 3, and at P in fig. 4. 



Directions for fixing Hie Barometer. In selecting a position for a barometer 

 care should be taken to place it so that the sun cannot shine upon it, and that 

 it is not affected by direct heat from a fire. The cistern should be from two to 

 three feet above the ground, which will give a height for observing convenient 

 to most persons. Having determined upon the position in which to place the 

 instrument, fix the mahogany board as nearly vertical as possible ; and ascer- 

 tain if the barometer is perfectly free from air, in the following manner : lower 

 the adjusting screw at the bottom of the cistern several turns, so that the 

 mercury in the tube, when held upright, may fall two or three inches from the 

 top ; then slightly incline the instrument from the vertical position, and if the 

 mercury in striking the top elicit a sharp tap, the instrument is perfect. If the 

 tap be dull, or not heard at all, there is air above the mercury ; this must be 

 driven into the cistern by partially rescrewing and then inverting the instrument, 

 and gently tapping it with the hand. The barometer being in perfect condition, 



