260 A. G. Howard. — The Barometer : [Sept. 28, 



the same trouble was experienced by using the temperature variable, 

 and in the year 1881 a system for all stations was introduced, by 

 using monthly constants of reduction to sea level based on monthly 

 normal pressures and temperatures, but this has features quite as 

 objectionable as any other mode. 



At the third meeting of the International Meteorological Committee 

 held at Paris in 1885, the late General Hazen of the American Signal 

 Service submitted a scheme for the reduction to sea level, by using 

 for the temperature variable, the mean of the observed temperature 

 and those of the two preceding observations where tri-daily obser- 

 vations are taken, and where only one observation a day is telegraphed 

 to send the mean of the hours 7, 2, 2 and 9. The idea a& far as t 

 can see is to use the progressive mean of the day. Mr.* Leyst*' 

 pointed out the error of this mode of reduction, which instead of- 

 obliterating the diurnal curve increased and misplaced it. He still 

 considers that the formula of La Place should be adhered to, and thG>-- 

 constants and variables revised according to the light of modern' 

 science, and furthermore that the formula should be critically examined- 

 with a view to constructing tables to avoid the use of logarithms. 

 He professes to have solved this problem himself. This may be j 

 but I do not believe, even at the best, that it will be reliable for great 

 heights. 



Rlihlmann, who has corrected the formula of La Place, makes the 

 temperature answerable for all errors of reduction and alters his 

 constants accordingly. But this, I think, is not a very scientific 

 proceeding. 



Leyst again points out that humidity has a considerable influence 

 on the pressure. So, I say, has the direction of currents, whether 

 they are rising or falling. The reductions in the centres of cyclones 

 and anti-cyclones would be totally different. 



Latitude also makes a difference. According to Leyst ; at an 

 altitude of 6,500 feet, a difference of 80? in latitude makes a difference 

 of 'OSd of an inch in the pressure. At the same altitude and with a 

 temperature of 20*^ centigrade, a difference in the relative humidity 

 of eighty-seven, causes a difference in pressure of '063 of an inch.. 

 Hence these fiactors are to be taken into account, and when we 

 consider that temperature and humidity vary considerably at different 

 heights, and that at a high station such variations are totally 



* Pawlowsk Observatory* 



