The Barometer in South Africa, 113 



It is tempting to find if the co-efficients of Tables III. and IV. 

 can be represented by a Fourier series. The partial results of such an 

 attempt will be found in Table V., in which 6 is measured from the 

 middle of January. The results are not satisfactory. The 

 co-efficients of the different terms vary so irregularly that it is plain 

 that the variations of the barometer are not simple functions of 

 the Sun's longitude. If we attempt to exhibit the equation 

 of time by a simple series in which the variable is the Sun's 

 mean longitude, we come on similar irregular co-efficients. It is, 

 of course, possible to exhibit the equation of time with great 

 exactitude by such a series, but it would throw no light on its causes, 

 which we know reside in the obliquity of the ecliptic, the excentricity 

 of the Earth's orbit and the position of the perihelion. But we do 

 not yet know all the causes of the diurnal variation of the barometer. 



There is the very great difficulty of finding the true height of the 

 barometer freed from temperature effects. The usual method of 

 correcting a barometer for temperature can only be considered a first 

 approximation. Probably each barometer has its own secondary 

 corrections due to lag, etc. Herein consists the great advantage of 

 the Sprung barograph, which records in effect the varying weight of 

 the mercury. It therefore requires no correction of the first order 

 for temperature, and the correction for terms of the second order is 

 easily made mechanically. 



It is impossible to say how much of the smaller terms of Table 

 III. are due to the residual temperature effects. 



