296 



NATURE 



[January 24, 1895 



its purpose,' yet the incidental discoveries, the import- 

 ance of which the author himself did not fully appreciate, 

 may perpetuate the name of Whiston for ever. It is most 

 remarkable that his contributions have been, apparently, 

 entirely overlooked. 



Whiston, who, as stated in the title of his book, was 

 at one time Professor of Mathematics at Cambridge, 

 being Sir Isaac Newton's successor, was banished not 

 long after assuming the chair, on account of heresy — 

 he was a Unitarian. He was led to pursue the longitude 

 problem magnetically through Halley's famous Isogonic 

 Chart of 1700," which came under his notice. As is well 

 known, this chart of Halleys, giving the lines of equal 

 magnetic declination, i.e. these lines on the earth's 

 surface connecting all the places at which a magnetic 

 needle swung horizontally has the same bearing, is the 

 earliest published chart of its kind. In consequence, 

 these lines have likewise been termed the '■ Halleyan 

 Lines." Since then Halley's method has been efi'ectually 

 applied to the representation of other terrestrial pheno- 

 mena, £\i,'. distribution of temperature (" Humboldt's 

 Isotherms," 1817). Wilcke is credited as first applying 

 Halley's method to the representation of the distribution 

 of the magnetic inclinalion, and Wilcke's isoclinics are 

 therefore referred to occasionally as the " Wilckean 

 Lines." Wilcke published his chart, covering the greater 

 portion of the earth, in 1768." It appears, however, that 

 the credit of first drawing the isoclinics should be 

 accorded to Whiston. Wilcke nowhere states in the 

 article cited that he for the first time has drawn the 

 lines of equal magnetic inclination, and it is, moreover, 

 reasonable to suppose, by his reference to Whiston's 

 book, that he was familiar with its contents. 



Whiston was led to drawing the isoclinics upon find- 

 ing that the " Halleyan Lines," through " the Quickness 

 of the Mutation of those Lines and their different Position 

 in the rest of the World," could not be satisfactorily used 

 for the determination of longitude. He therefore began 

 to consider the "lines of equal dip,"' thinking they 

 would answer his purpose better. To this end he col- 

 lected all observations of dip made up to his time, and 

 with their assistance drew, as far as then possible, the 

 "lines of equal dip" upon Mr. Molyneux's terrestrial 

 globe. Furthermore, to practically test his method, 

 he made dip observations himself in I7i9and 1720 in 

 various portions of England, and with their aid drew 

 and published the firsl isoclinics, to be sure for only a 

 small portion of the earth, viz. for southern England 

 and north-western France. These isoclinics were laid 

 down on two small charts {w\y. 182 cm.), and are 

 given opposite p. xxviii. of his book. The first chart is 

 the result of dip observations made in 17 19 with a 

 needle 12 inches long; the second '■' is based upon more 

 numerous observations made in 1720 with a 47J,-inch 

 needle. The results with the two needles differ, on the 

 average, by about i). , the long needle giving the larger 

 value. To counteract the error due to flexure of the long 

 needle, Whiston pl.iced a small " Poise of brass circular 

 Wire," whrch required shifting to and fro according to 

 dip, on the north end of the needle. Whiston believed 

 that the longer the needle, the better the result if the 

 needle be poised as stated. It is needless to say that 

 experience has not borne him out in this respect. Owing 



* When Graham diwrivemd, a few yean afler the publication of Whiiton's 

 book. tltA* i^rr^ 'ri.l ina|{nctt*m ift Rubject to a daily variation, Wtitston 

 'if hi* method. See ** Meriioir*i of the Life and 

 11 WhiMon. Written by himwlf." (London, 1749, 



lith'iitr.nphicatly in 

 • '1 tn '* N'cudriicke 

 ' < ■ Kr(lm;4f[ncliitniliA." 

 \ f rier and Co.) 



: .u ■ . Ic ParalIeK" 



■I \>'. Keii[€n[rati;rr'^paper('M:c Ref. 3). Both 

 . factimjj?, in Hellniann'-i " Netidniclce," No. 



Gr- 



E'l 



to the large discrepancy between the results of the two 

 needles, it might appear, then, that but little value can 

 be attached to these Whistonian isoclinics. The writer 

 finds, however, in his paper that the mean of the two 

 results can doubtless be accepted as being within i" of 

 the truth. Moreover, while the absolute value may be 

 impaired to the extent mentioned, the relative value 

 remains intact, as the elTect of the constant instrumental 

 error would be almost entirely eliminated when consider- 

 ing the relative distribution of the dip over England. 

 It is this latter fact that may give a value to these early 

 isoclinics. Thus in 1720, according to Whiston, the 

 isoclinics over England ran approximately from west- 

 north-west to east-south-east, while to-day they go 

 roughly from west-south-west to east-north-east. In the 

 accompanying sketch, the mean isoclinics are shown by 

 full lines for the epochs 1720, 1837, 1S60, and 1886, as 

 drawn by Dr. Felgentraeger in the paper cited.' It 

 will be seen that between 1720 to about 1S37 they shifted 

 from year to year, anti-clockiiiise ; they are now moving 

 clockwise. Sabine,'" I believe, was the first to call 

 attention to this reversal of motion. It is hoped that 

 this matter will receive further investigation. 



ch. 

 4(. 



NO. r;i 



VOL. 



51] 



n'itttfr t'^ j;,uita/t if. 



Whiston comes in, however, for a still ;4reater share in 

 the early terrestrial magnetic discoveries. He invents, 

 namely, a new and indirect method of determining the 

 dip — the vibration method. He swings a magnetic 

 needle horizontally, and determines the time of one 

 horizontal vibration. He then swings the same needle 

 mounted as a dip-needle, and again determines the time 

 of one vibration. From the two times of vibration it is 

 a simple matter to compute the ])rcvailing dip. Whiston 

 proposes this method as an approximate check upon the 

 direct method where the angle of dip is measured 

 at once. For example, Whiston found at London the 

 time of one horizontal vibration of his long needle 120' 

 from the magnetic meridian to be 60.} seconds, which 

 reduced to the magnetic meridian gives 426 seconds. 

 The time of one vibration of the same needle, mounted 

 as dip-needle, was found to be 22 seconds. If F is the 

 total magnetic force, we then have the following relation: 

 F : F cos Dip = (42-6)= : (22)'-, or Dip = 74° 32'. 



Now, the mean of the results of the direct measure- 

 ments with the 1 2-inch needle (73? ), and of the 47l-inch 



'<* /V(V. ffa^. Soc.t vol. xi. p. 144 : " The anjile of intersection of the 

 meridian and isoctinic* han been diminishing lip tn .iboiit 1840. when a rc- 

 vemal took place, .ind the nniile i^ now incrcasinc." 



