CLIMATES OP GREAT MALVERN AND LONDON. 71 



taken from the scale you intend to apply to your barometer, — ^mea- 

 sure it two or three times, to be perfectly correct ; put a minute 

 speck of ink at each end of the measured distance ; with a fine file 

 make a mark upon the tube, about an inch or an inch and a half 

 from the end now closed with sealing-wax ; place this in exact op- 

 position with one of the ink marks upon the paper, and laying the 

 tube along the pencil line, make a little mark upon it by a touch of 

 the file, exactly corresponding with the other ink mark upon the 

 paper. By repeating this operation, in the most careful manner, you 

 will have 28 inches accurately measured upon the tube. Then 1st. 

 by adapting an ivory point to the cistern, the extremity of which 

 must exactly correspond to the lower file-mark upon the tube, — 2nd. 

 fixing the 28th inch division of the scale opposite the uppermost file 

 mark, — and 3rd., keeping the surface of the mercury in the cistern 

 always just touching the ivory point, the indications of your baro- 

 meter may be fully relied on.* The construction of the cistern is 

 described in the second volume of " The Analyst j' p. 217- 



After the cistern has been fixed, and before entirely closing the 

 bottom of it, of course, the sealing wax and gummed paper must be 

 removed from the end of the tube. 



With regard to Thermometers. For the purposes before stated 

 three will be necessary: one — a self-registering, of Rutherford's 

 construction — for the minimum ; another self-registering for the 

 maximum ; and a third to hang close by the barometer. These 

 should be purchased of some philosophical instrument maker, on 

 whom reliance can be placed ; where, also, the Hygrometer should 

 be obtained. 



• The corrections for the temperature of the mercurial column will be 

 found in a table, p. 372, and for the bore of the tube, p. 363, of Daniell's Es- 

 says. And when these are applied, the constant difference between the mean 

 of any two barometers, within short distances, is attributable to elevation. 



