ON THE MAGNETIC SURVEY OF ENGLAND. 271 



tively to the force in London expressed by 1-0000 ; each observer taking some 

 spot in the immediate vicinity of London as his base station, and thus ren- 

 dering the results of all the observers intercomparable. The stations, their 

 latitudes and longitudes, the initial denoting the observer, and the observed in- 

 tensity of the force, are collected in the following Table (XV.) ; the initials 

 F., L., P., R., and S. refer respectively to Mr. Robert Were Fox, Dr. Lloyd, 

 Professor Phillips, Sir James Clark Ross, and myself. Adopting the same 

 central position as that of the present survey, viz. 52° 13' N. and 1° 38' W., we 

 have the coordinates of distance a and b as shown in Table XV., and combining 

 these data in the customary manner, we obtain 1-0051 as the representative 

 value of the whole series at the central position expressed in terms of its own 

 arbitrary scale. The force in absolute measure at the same central position 

 derived from the experiments of the present survey is 10-332 in British units. 

 Omitting for the present the consideration of any secular change which may 

 have taken place in the value of the force in the interval between the two 

 surveys, we are thus furnished with the means of expressing the results of the 

 first survey in terms which bring them in immediate comparison with those 

 of the second survey. The absolute values thus obtained for the several sta- 

 tions of the first survey are placed in column 8 of Table XV., and being com- 

 bined in the manner described in pp. 99-101 of the British Association Report 

 for 1836, we obtain for the survey of 1837, a-= -f -00052 1, y =+-000742, 

 M=_54.°52', and r=-0009; and the formula for computing the force iu 

 different parts of England corresponding to the observations of the same 

 survey is 0= 10'332+ -000521 « + '000742 6, 



a and 5 being coordinates of the distance from the central position in 1° 38' W. 

 and 52° 13' N., expressed in geographical miles. The several values thus 

 computed are placed in column 9, and the differences between the computed 

 and observed values in column 10. From the latter we find the probable 

 error of a determination at a single station = +-012. (Table XV. see p. 272.) 



The direction of the isodynamic lines in England thus found for the epoch 

 of the present survey is from N. 57° 35'-5 E. to S. 57° 35'-5 W. The direc- 

 tion found by the preceding survey was from N. 54° 54' E. to S. 54° 54' W. ; 

 the central position being the same in both surveys. It appears, therefore, 

 that the isodynamic lines passing across England have increased the angle 

 which they make with the geographical meridian in the interval between the 

 two surveys. The change is similar in character to the change in the direc- 

 tion of the isoclinal lines in the same interval, but somewhat less in amount. 



In the survey of 1837, the rate of increase of the magnetic force in each 

 geographical mile towards the N.W., measured in the direction perpendicular 

 to the isodynamic lines, was -00091 ; in the survey of 1860 -00106. There- 

 fore in the interval the increase of the force towards the N.W. had become 

 more rapid, and the isodynamics corresponding to equal increments of force 

 more closely packed. Hence we may infer that in the northern parts of 

 England the secular increase of the force had been greater than in the 

 southern parts. 



Plate IX. exhibits the fundamental lines of the two surveys in comparison 

 with each other. Both pass through the central position in lat. 52° 13' N. 

 and long. ]°38'W.; that of 1837 at an angle of —54° 54', and that of 1860 

 at an angle of 57° 35'-5. Each has the value of 10-332 in British units ; 

 which is accurate for 1860, being subject only to errors of observation or of 

 the hypothesis by which the observations are combined; but in 1837 is less 

 certain, because no account is taken of the secular change which may have 

 taken place in the absolute value of the force in the interval between the 



