270 



REPORT — 1861. 



normal direction ="00102 (in British units) for each geographical mile. The 

 formula for computing the total force at each station ((p) is 

 ^=10-332 + -000557 a+ -000878 b, 



a and b being coordinates of the distance of the station from the central 

 position, expressed in geographical miles. 



The observed and computed values of the force at the several stations are 

 shown in Table XIV. 



Table XIV. 



The " probable error" at a single station is +-014. 



In the map of the isodynamic lines accompanying this survey, the line of 

 10-332 (in absolute measure, British units) is distinguished by a strong un- 

 broken line passing through the central station in lat. 52° 13' and long. 

 1° 38' W. ; the lines of 10-200, 10-300, 10-400, 10-500, and 10-600 being re- 

 presented by fainter but also unbroken lines. All are computed by the formula 

 <l> = 10-332 + -000557 a + -000878 b, 



for the intersections of the isodynamic lines with the meridians of 2° E., 1° E., 

 0°, 1° W., 2° W., 3° W., 4° W., 5° W., and 6° W. 



We have now to compare with these the lines determined by the previous 

 survey in 1 837. The general Table of the results obtained in that survey by the 

 statical method are given in pp. 190, 191, and 192 of the Report of the 8th 

 (Newcastle) Meeting of the British Association in August 1838; it contains 57 

 determinations of the total force at stations in England, by five observers acting 

 independently of each other, but adopting the same general principle of ex- 

 periment. The method of determining the value of the force in absolute 

 measure had not then been introduced, and the values of the force at the 

 different stations are expressed in that Table (as was then the custom) rela- 



