56 



GREEN'S STANDARD BAROMETER. 



with the glass tube, indicates the temperature of the mercury in 

 the barometer tube, not that of the external air. This central 

 position of the thermometer is selected that the mean tempera- 

 ture of the whole column may be obtained; a matter of import- 

 ance, as the temperature of the barometric column must be taken 

 into account in every scientific application of its observed height. 

 The cistern (Fig. 2) is made up of a glass cylinder F, which 

 allows the surface of the mercury q to be seen, and a top plate G, 

 through the neck of which the barometer-tube t passes, and to 

 which it is fastened by a piece of kid leather, making a strong 

 but flexible joint. To this plate, also, is attached a small ivory 

 point h, the extremity of which marks the commencement or 

 zero of the scale above. The lower part, containing the mer- 

 cury, in which the end of the barometer-tube t is plunged, is 

 formed of two parts * j, held together by four screws and two 

 divided rings I m, in the manner shown in the Figures 2, 3, and 



4. To the lower piece j is 

 fastened the flexible bag J\ 7 , 

 made- of kid leather, furnished 

 in the middle with a socket k, 

 which rests on the end of the ad- 

 justing-screw 0. These parts, 

 with the glass cylinder F, are 

 clamped to the flange B by 

 means of four long screws P 

 and the ring R ; on the ring R 

 screws the cap S, which covers 

 the lower parts of the cistern, 

 and supports at the end the 

 adjusting-screw 0. G, i, j, 

 and k, are of boxwood ; the 



other parts of brass or German silver. The screw serves to 

 adjust the mercury to the ivory point, and also, by raising the 

 bag, so as to completely fill the cistern and tube with mercury, 

 to put the instrument in condition for transportation. 



In Fortin's barometer, and also Delcro's modification of it, a 

 cement is used to secure the mercury against leakage at the 

 joints. This, sooner or later, is sure to give way ; and tested 

 under the extremes of the thermometrical and hygrometrical 

 range of this climate especially, has made this defect more evi- 



