148 SCIENCE IN SHORT CHAPTERS. 



5th. A fluctuating and unsettled state of the mercurial 

 column indicates changeable weather. 



As the barometer is subject to slight diurnal variations, 

 irrespective of those atmospheric changes which affect the 

 weather, it is desirable in making comparative observations 

 to do so at fixed hours of the day. Nine or ten in the 

 morning and same hour in the evening are good times for 

 observations that are to be recorded. These are about the 

 hours of daily maxima or highest readings due to regular 

 diurnal variation. 



The true reading of the barometer is the height at which 

 it would stand if placed at the level of the sea at high tide; 

 but, as barometers are always placed more or less above 

 this level, a correction for elevation is necessary. When 

 the height of the place is known this correction may be 

 made by adding one tenth of an inch to the actual reading 

 for every 85 feet of elevation up to 510 feet; the same for 

 every 90 feet between 510 and 1.140 feet, for every 95 feet 

 between 1140 and 1900 feet, and for every 100 feet above 

 this and within our mountain limits. This simple and 

 easy rule is sufficiently accurate for practical purposes. 

 Thus, a Jbarometer on Bray Head, or any place 800 feet 

 above the sea, would require a correction of six- tenths for 

 the first 510 feet, and a little more than three-tenths more 

 for the remaining 290 feet. Therefore, if such a barome- 

 ter registered the pressure at 29 T V, the proper sea-level 

 reading would be a little above 30 inches. 



The most important prognostications of the barometer 

 are those afforded by what is called the "barometric gra- 

 dient or incline," showing the up-hill and down-hill direc- 

 tion of the atmospheric inequalities; but this can only be 

 ascertained by comparing the state of the barometer at "dif- 

 ferent stations at the same time. Thus, if the barometer is 

 one-fourth of an inch higher at Dublin than at Galway, 

 and the intermediate stations show intermediate heights, 

 there must be an atmospheric down-hill gradient from 

 Dublin to Galway; Dublin must be under the upper and 

 Galway under the lower portion of a great atmospheric 

 wave or current. It is evident that when there is thus 

 more air over Dublin than over Galway, there must follow 



