ON THE MAGNETIC SURVEY OF ENGLAND. 261 



mit ourselves to extend the same scale of variation to the north of Scotland, 

 we should find the mean annual decrease reduced to l'-6. The very small 

 corrections required to reduce the results of the observations of the present 

 survey to a common epoch (January 1, 1860), as shown in Table II. p. 257, 

 have been estimated in accordance with this scale of variation : the whole of 

 the observations were made within two years of the common epoch. 



The line which has been indicated as connecting the intersections of the 

 isoclinals of 68°, 69°, and 70° of 1837 with those of 69°, 70° and 71° of 1860, 

 is marked on the map by a faintly dotted line. It is in fact a line com- 

 posed of nodal points, on which the isoclinals passing through them may be 

 conceived to have turned, as on pivots, in the interval of 23 years, and 

 (irrespective of their common and uniform movement of translation to the 

 north) to have undergone a change of direction, becoming more southerly on 

 the eastern side of the nodal line, and more northerly on its western side. 



Division II. — Intensity of the Magnetic Force. 



For the purpose of ascertaining the position, direction, and distance apart 

 of the isodynamic lines, or lines of equal Total Force, two methods were 

 employed, viz. (a) the determination at different stations of the values in 

 absolute measure of the horizontal component of tlie force, which values, 

 being combined with the dip of the needle observed at the same time and 

 place, give the absolute values of the total force; and (b) the determination 

 of the variations of the total force itself at the different stations, by observing 

 the positions of equilibrium of a dipping-needle between the action of the 

 earth's magnetism and tiiat of a small constant weight with which the needle 

 is loaded. It may be convenient to discuss these methods and their results 

 separately ; and with this view we may commence with the determinations 

 of the absolute value of the Iiorizontal component of the force. 



a. Horizontal Force in Absolute Measure. — A full description of the ia- 

 "struments, and of the method employed in these experiments, is given in 

 App. I. of the article on " Terrestrial Magnetism " in the 3rd edition of the 

 * Manual of Scientific Inquiry,' published under the authority of the Ad- 

 miralty. The collimator magnet employed as a deflector was niniibered 5, 

 and was used throughout the experiments. Its moment of inertia (K), in- 

 cluding the suspending stirrup and other appendages, was determined at 

 Kew, by the late Mr. WelsJi, in June 1858, by the mean of experiments 

 with three cylinders B, C, and D, of which the weights and dimensions were 

 respectively as follows — 



in. in. grs. 



B length 4-0193 ; diam. 0-3917 ; weight 1029,62, 



C length 4-0488; diam. 0-3929 ; weight 1044,42, 



D length 4-0131 ; diam. 0-3916 ; weight 1029,71, 



whence K was found =0-73100 at 60° Fahr.; and the log of 7r'K = 



1-72513 at 30° Fahr. 1-72531 at 60° Fahr. 1-72549 at 90° Fahr. 

 1-72519 at 40° Fahr. 1-72537 at 70° Fahr. 

 1-72525 at 50° Fahr. 1-72543 at 80° Fahr. 



The correction for the decrease of the magnetic moment of No. 5, produced 

 by an increase of 1° Falir.=(9)=0-00011 (^„—<) + '0000006 {t^—tf,t^ being 

 the observed temperature, and ^=45°. The induction coefficient (^t) =0-000252. 

 These were both determined at Kew by the same careful experimentalist. 

 The angular value of one scale-division of the vibration apparatus=2'-27. 

 The graduation of the deflection bar, compared with the verified standard 

 measure of the Kew Observatory, was without error within the limits which 

 were used. The rate of the chronometer and the arc of vibration were too 



