northern and southern parts of England which is implied thereby may be similarly shown, by comparing the dips in 1837 and 1860 at two geographical positions, one in the extreme north, and the other in the extreme south of England. Taking as the northern station the intersection in the map of January 1837 of the isoclinal line of 71° 30' with the parallel of 55° N. latitude, which takes place in the longitude of 3° 00′ W.,-and for the southern station the intersection of the isoclinal line of 69° in the same map with the parallel of 51°, which takes place in 0° 07' East longitude,-and comparing these values with the values computed for January 1860 by the formula corresponding to that epoch, we have The comparison shows that the mean annual secular change in the interval of 23 years between the surveys was 2'05 on the northern border of England, and 2'68 at a station on the south coast. Thus it is seen that the annual rate of decrease of dip has varied in different parts of England in proceeding from east to west, from 2'36 at Lowestoft to 3'09 at the Land's End; and in proceeding from north to south from 2'05 at a position in 55° to 2'68 at a position in 51° N. latitude. In viewing the map in which the isoclinal lines for 1837 and 1860 are represented in comparison with each other (Pl. VIII.), it is seen that there are three points where the amount of secular change in the interval must have been the same, viz. the three points where the lines of 68°, 69°, and 70° in 1837 intersect respectively with those of 69°, 70°, and 71° in 1860; since at each of these points the mean annual change must have been (60'÷23 years)2′·62. These three points are seen to be in a curved line which crosses England from the vicinity of Folkestone to the Irish Channel, and would impinge upon the east coast of Ireland a few miles north of Dublin. Kew, also, where the mean annual decrease of the dip in the same interval has been 2'63, is as nearly as may be on the same line. At all stations north and east of the line the mean annual secular change in the 23 years has been less than 2'-62, and at all stations south and west of the line greater than 2'-62. In a preceding page we have the mean annual change at four stations (Lowestoft, Margate, Lew Trenchard, and Land's End) situated at points on the east and west sides of England, and at two geographical positions (55° N. and 3° W., and 51° N. and 0° 07′ E.) at north and south points. An intercomparison of the respective values of annual change at these six localities with 2'62, and of their geographical distances from the aforesaid line of 2'62 measured in every case on a perpendicular to that line, shows that an increase of O'-1 in the annular secular change for every 30 geographical miles towards the N.E., and a decrease of O'1 for every 30 geographical miles towards the S.W., will represent very approximately the observed values. We are thus furnished with a scale by which the variation in the mean rate of the secular decrease of the dip in different parts of England in the interval between the two surveys may be approximately assigned; the limits being an annual decrease of 3'.1 at the Land's End, and of 2'0 at Berwick. If we should per mit ourselves to extend the same scale of variation to the north of Scotland, we should find the mean annual decrease reduced to 1'6. 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 composed 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 the 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 that 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 horizontal component of the force. a. Horizontal Force in Absolute Measure.--A full description of the instruments, 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 Admiralty. The collimator magnet employed as a deflector was numbered 5, and was used throughout the experiments. Its moment of inertia (K), including the suspending stirrup and other appendages, was determined at Kew, by the late Mr. Welsh, 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 B C D ...... ...... in. in. grs. length 4-0193; diam. 0.3917; weight 1029,62, whence K was found =0·73100 at 60° Fahr.; and the log of 2K= 1.72513 at 30° Fahr. 1.72519 at 40° Fahr. 1.72525 at 50° Fahr. 172531 at 60° Fahr. 1.72549 at 90° Fahr. 172537 at 70° Fahr. 172543 at 80° Fahr. The correction for the decrease of the magnetic moment of No. 5, produced by an increase of 1° Fahr.=(q)=0·00011 (t−t)+0000006 (t−t), to being the observed temperature, and t=45°. The induction coefficient (u)=0000252. 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 small throughout the experiments to require corrections to be applied on their account. The constant P, depending upon the distribution of magnetism in the deflecting and suspended magnets (the same magnets having been used throughout), was determined by the experiments in Table IV. made at Kew by Mr. Chambers: TABLE IV.-Deflections with Collimator 5 at distances 0-9 ft. and 1.2 ft. to determine the value of P. Observed by Mr. Chambers. Distance o'9 ft.=r; 1+3 2100069;t=45°. Distance 1-2 ft.=r1; 1+2=1'00029; 1=45° The experiments detailed in Tables V. and VI. were made with Collimator No. 5, in June 1858, and in January and March 1859, by Messrs. Welsh and Chambers, to determine the value in British units of the magnetic force at the Kew Observatory, adopted as the base station of the Survey. Ø in June 1858 from 115 observations at Kew (68° 23'-2 on July 1, 1858). |