METEOHOLOGY. 



GREEN'S STANDARD BAROMETER. 



The following is an account of Green's improved standard barometer, 



adopted by the Smithsonian Institution, for observers of the first class. 



The barometer consists of a brass tube, (Fig. 1) terminating at 



top in a ring A, for suspension, and at bottom in a ilange B, to which 



the several parts forming the cistern are attached. 



The upper part of this tube is cut through so as to expose the glass 



tube and mercurial column within, seen 

 in Fig. 5. Attached at one side of this 

 opening is a scale, graduated in inches 

 and parts ; and inside this slides a short 

 tube c, connected to a rack-work ar- 

 rangement, moved by a milled head D : 

 this sliding-tube carries a vernier in 

 contact with the scale, which reads oif 

 to -j^Q- (-002) of an inch. 



in the middle of the brass tube is 

 fixed the thermometer E, the bulb of 

 Mihich being externally covered, but in- 

 wardly open, and nearly in contact with 

 the glass tube, indicates the tempera- 

 ture of the mercury in the barometer 

 tube, not that of the external air. This 

 central position of the thermometer is 

 selected that the mean temperature of 

 the whole column may be obtained ; a 

 matter of importance, as the tempera- 

 ^ ture 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 sur- 

 face of the mercury q to be seen, a top- 

 plate Gr, 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 li, the extremity of which 

 marks the commencement or zero of the 

 scale above. The lower part, contain- 

 ing the mercury, in which the end of the 

 barometer-tube t is plunged, is formed of two parts ij, held together 

 by four screws and two divided rings I on, in the manner shown in the 



