THE BAROMETER : ITS REDUCTION TO SEA LEVEL. 



By a. Gt. Howard. 



[Read 28th September, 1887.] 



The subject of my paper is the reduction of an indicated 

 atmospheric pressure at an elevated station to that which it would be 

 were the earth below that station removed, and the observation of 

 pressure made at the sea level. The object of thus reducing the 

 indicated pressures to one datum, is for the purpose of charting a 

 number of simultaneous observations for comparison and for the 

 drawing of isobars or lines of equal pressure. The datum employed is 

 .usually the mean sea level. 



The true formula for this reduction has not yet been developed, and 

 lit is my firm opinion, that as far as reductions of pressures on isolated 

 i mountains are concerned no formula will ever be produced for giving 

 the true sea-level equivalent. The local eddies, and alterations of 

 temperature, pressure and humidity are such, in such a position, not 

 only at the station itself but at varying heights, that the formula 

 which would be correct at one time would give very erroneous results 

 at another. 



But on elevated table lands the conditions are totally different ; in 

 such a position there are no local eddies whirling up or down, no 

 atmosphere of varying temperature and humidity below ; whatever 

 the reduction is calculated to be, it can only vary with the pressure ; 

 the temperature correction has already been applied to the column of 

 air above the station ; consequently the conditions are very much 

 jsimplified. 



The formula of La Place takes into consideration the mean tem- 

 perature of the column of air below the station, but this is not always 

 so easy to find. The actual mean is not always the mean of the two 

 •stations. The temperature variable makes a great difference in the 

 reduced reading, and is often very misleading ; so much so was this 

 the case, that in the year 1872 the Chief Signal Officer in America 

 .ceased to use this variable for high stations and substituted an annual 

 constant. Bui still at stations of from 2,000 to 5,000 feet in altitude, 



