112 Report S.A.A. Advancement of Science. 



represents the mean monthly height of the barometer at St. Helena as 

 follows : — 



As the average deviation of any given month is about o.oi2in. from 

 the mean of a series of years, the above representation is satisfactory. 



In considering the diurnal variation of the barometer at St. Helena 

 we have confined ourselves to the period during which hourly readings 

 were taken. Allowance has been made for the difference of height 

 in the mercury between the first and last readings in each month, so 

 that the annual variation of the barometer has been eliminated. After 

 the calculations had been completed, it was found that Mons. Angot 

 had also derived the figures. A comparison with Mons. Angot's 

 table {Annates dii Bureau, 1889, Vol. I., P.B250) shows that our 

 figures, though in the main following his very closely, sometimes 

 deviate by quantities up to 0.005 ins. These differences may arise 

 from two causes, (i) in the published observations a considerable 

 number of errors or misprints were found, and it is probable others 

 have escaped detection by either of us, (2) Mons. Angot probably 

 incorporated the two-hourly series. 



If we represent the deviation at any hour from mean of day by 

 the series 



Ci cos h-fCo cos 2h-TC3 COS 3h + etc. 

 -f Si sin h + So cos 2h + S3 sin 3h+etc. 



we find the values given in Table III. 



The above formula can be transformed to 



a, sin (h+A2)+a2 sin (2h+A2)+etc. 

 The co-efficients in this case are furnished in Table IV. Mons. 

 Angot gives the latter co-efficients up to a, and A4 reduced to true 

 time. The reduction to true time may amount to 15° in the case of 

 A.. 



A priori, the introduction of true solar time seems to be correct 

 in principle, but the figures do not show improvement. The angle A2 

 is the most constant ; in Table IV. it shows an extreme range of 9°, 

 whereas by reducing to true time, the range will be increased to 13°. 



