DISCUSSION OF BAROMETRIC OBSERVATIONS. 



4^3 



ANNUAL FLUCTUATION OF ATMOSPHERIC PRESSURE. 



Ill order to deduce the annual march of atmospheric pressure with as 

 much accuracy as the Amherst twenty-hve-year series renders possible, 

 the first step lias been to derive the coefficients of Bessel's periodic func- 

 tion from tlie given monthly means. These, as given in the records, 

 were of course originally derived from the daily means, and these last 

 iire simply the third part of the sum of the three daily observations, no 

 additional weight being given to the evening observation. In the re- 

 duction of the barometric observations no correction for hours of observa- 

 tion has been attempted, but a correction for the unequal length of the 

 calendar months was made in the manner presented by Mr. E. L. De 

 Forest, and quoted on page 170 of Mr. Charles Schott's paper on tem- 

 perature {Smithson. Conf., vol. XXI). The monthly means thus corrected 

 are as follows : * 



January 29. 7742 



February 29. 7354 



March 29.6717 



April 29. 6776 



May 29.6588 



June 29. 6815 



July 29. 7065 



August 29. 7257 



September 29. 7968 



October 29. 7500 



November 29.7071 



December 29.7626 



The form of the function derived in this i^reliminary computation is 

 the following: 



P = 29.7207+.0467 sin (^+142O20') + .02-J7 sin (2^+10O34') + .0241 sin 

 (3 ^4. 440 41/) 4. .0139 sin (4^+92044'). 



From this formula a table of normal pressure for every day of the 

 year was prepared, the value of P being computed directly from the 

 formula for March 1st, and every tenth day thereafter to August l»3th; 

 also for September 1st and every tenth day to February 28th. The 

 values for the intermediate days were then obtained by interpolation, 

 using second differences, except a few days at critical epochs, the values 

 for which were again computed from the formula. The table of normal 

 pressures thus deduced is as follows, the integral i)art of each value in 

 this and succeeding tables of barometric pressures being omitted for 



*The amount of this correction for length of months maybe seen by comparing the 

 means here given with those appended to Table I in the line headed " Means from Rec- 

 ord." I did not think it necessary to verify the work of Professor Snell in obtaining 

 the means of the single months from the daily means or the latter from the observed 

 heights, especially as the formula resulting from the present computation is used only 

 to direct the subsequent work. A separate determination of the mean monthly pres- 

 sures, independent of the recorded monthly means, occurs at the end of Table II. 

 Since my work was completed, however, I have received from H. A. Hazeu, M. A., a 

 list of errors in the recorded monthly means which he has discovered in examining 

 Professor Snell's work. Making allowance for these errors the mean monthly pres- 

 sures, uncorrected for unequal lengths of the months, are as given in the last line of 

 Table I, " Means from corrected record." In Table VI (the only remaining part of my 

 work which is derived from the recorded monthly means) I have substituted Mr. 

 Hazen's determinations wherever he has found those of Professor Snell erroneous. 



