108 THIRD REPORT — 1833. 



entitled to much confidence ; but he was of the same opinion 

 as Norman with respect to the constancy of the variation*. 

 Gunter, in 1612, found the variation in London to be 5° 36' 

 east; and Gellibrand, in 1633, observed it 4° 4' east. Dr. Wal- 

 lis considers Gellibrand to have been the discoverer of " the 

 variation of the variation f ; " but if Gunter had any confidence 

 in his own observations and those of Burough, he must have 

 been aware of the change in the variation. In 1630, Petit 

 found the variation at Paris to be 4^° east, but suspected, at 

 the time, that the earlier observations there had been incorrect; 

 and it was not until 1660, when he found the variation to be 

 only 10' east, that he was satisfied of the change of the varia- 

 tion. About ten years later, Azaut, at Rome, where the va- 

 riation had been observed 8° east, found it to be more than 2° 

 west; and Hevelius, who at Dantzick in 1642 had found it to 

 be 3° 5' west, now found it to be 7° 20' west. 



3. Diurnal Change in the Variation. — This was discovered 

 in 1722 by Graham, to whose talents and mechanical skill 

 science is so deeply indebted. He found that with several 

 needles, on the construction of which much care had been be- 

 stowed, the variation was not always the same ; and at length 

 determined that the variation was different at different hours 

 in the day, the greatest westerly variation occurring between 

 noon and four hours after, and the least about six or seven 

 o'clock in the evening f. Wargentin at Stockholm in 1750, 

 and Canton in London from 1756 to 1759, moi-e particularly 

 observed this phsenomenon; and the latter determined that 

 the time of minimum westerly variation in London was between 

 eight and nine in the morning, and the time of maximum be- 

 tween one and two in the afternoon. Canton likewise deter- 

 mined in 1759, that the daily variation was different at different 

 times in the year, the maximum change occurring about the end 

 of June, and the minimum in December §. Cassini, during more 

 than five years and a half, namely, from May 1783 to January 

 1789, carefully observed, at particular hours, the direction of a 

 needle suspended in the Observatory at Paris ; and although 

 he does not correctly state the course of the daily variation, 

 overlooking altogether the second maximum west, and the pro- 

 gress of the needle towards the east in the early part of the 



* " For considering it remayneth alwaies constant without alteration in every 

 severall place, there is hope it may be reduced into method and rule." Dis- 

 course, chap. X. 



f Philosophical Transactions, 1701, vol. xxii. p. 10-36. 



+ Ibid. 1724, vol. xxxiii. p. 96. 



§ Ibid. 1759, vol. xli. p. JWS. 



