45, COBNF1ILL, E.G., AND 122, BEGENT STREET, W., LONDON. 139 



PEDIMENT BAROMETERS. 



NEXT to a Standard instrument the Pediment Barometer must be regarded in 

 a scientific point of view as the most accurate form of Barometer, the actual 

 weight or pressure of the atmosphere being exhibited by the varying length of 

 the mercurial column itself, subject to a few corrections that need not be noticed 

 by ordinary observers. At pages 2 and 3 will be found the general principles 

 of the straight tube or Pediment Barometer. 



The cistern of the Pediment Barometer is made of boxwood, with sufficient 

 internal area to allow of a fall of at least two- thirds of the mercury contained 

 in the tube when the Barometer is in action without materially interfering with 

 the correctness of the readings. It should also contain sufficient mercury to 

 prevent air passing up into the tube. The bottom of the cistern is formed of 

 flexible leather, so as to admit of the use of a screw to render the Barometer 

 portable, as described in the paragraph Standard Barometer, pages 4 and 5. 



It will be seen that most of these Barometers are furnished with two Verniers, 

 or indices. The use of the second Vernier is to record on the left hand scale of 

 the instrument the previous reading of the Barometer, and show at a glance 

 any alteration that may have taken place by the difference of the readings of 

 the two Verniers. 



In taking a reading or observation by the Pediment Barometer the Vernier 

 carrying the Index Pointer is to be moved gently up or down, until its edge is 

 exactly in a line with the centre of the top of the mercurial column as shown 

 in fig. 5, page 6. If when adjusted the edge of the index is exactly in a 

 straight line with, say, the division marked 30, then the height or length of the 

 mercurial column is exactly thirty inches. The value of this column is given on 

 pages 4 and 5 ; also on pages 6 and 7 will be found a description of the use 

 of the Vernier, especially at the foot of page 7, where the Vernier of the 

 ordinary Household Barometer is spoken of as subdividing the inch scale into 

 hundredths. 



If the division 1 in the Vernier coincides with the line at 29 inches on the 

 scale, then the reading would be 29'11; if division 2 coincides with the line 

 below that marked 29 inches, then the reading would be 29'12 ; that is twenty- 

 nine inches and eleven hundredths or twenty-nine inches and twelve hundredths, 

 or it may be read twenty-nine inches one tenth and one hundredth, and so on. 

 The allowance to be made for height of the Station above the sea-level is, as 

 stated by Admiral Fitz-Roy, as under. 



The average height of the barometer, in England, at the sea-level, is about 

 29*94 inches, and the average temperature of air is nearly 50 degrees. 



Every ten feet of elevation above the sea lowers the Barometer about ten 

 or eleven thousandths of an inch. 



Add one-tenth of an inch to the observed height for each hundred feet the 

 Barometer is above the mean sea-level. This sea-level should be that of the 

 ocean itself, at mean half-tide, a level which should be the universal standard 

 line of reference. 



The Thermometer falls about one degree for each three hundred feet of 

 elevation above more than fifty feet from the ground. 



