.1820.] retrograde Variation of the Compass. 339 



IP 15' E. ; but as it. is not stated whether this was its greatest 

 excess, the discovery of the maximum of eastern variation must 

 be left to the assiduity of future observers. 



As the observations at this place prove that, the needle is now 

 retrograde, it is to be regretted that others, with proper instru- 

 ments, have not been made in different parts or the world, as 

 they would clear up a point, at present questionable, whether 

 the increase and decrease of the variation is simultaneous in 

 every part of the globe. 



The only observations which, 1 believe, have been published 

 are those of the Royal Society, commenced by the late Mr* 

 Gilpin, and continued by the present librarian ; but notwith- 

 standing the accuracy of the former, and the well-known 

 scientific abilities of Mr. Lee, these observations being made in 

 a room in which iron has been used to strengthen the ceiling: 

 (and not in the open air), it is doubtful whether the real varia- 

 tion can be truly ascertained. Magnetical observations have, I 

 believe, lately commenced in two places ; at the observatory of 

 Glasgow, and at Chatiilon, in Burgundy. Monsieur le Due de 

 llagusa, under whose auspices the latter are conducted, instead 

 of allowing the needle to turn on a point, suspends it by a thread 

 (nl) \ but what advantages attend this alteration remain to be 

 proved. 



Two needles were always used for finding the variation ; when 

 one set of observations was made, the needle was unhung, 

 replaced by a second, and a new series commenced ; and when 

 both v>ere completed, the mean result was considered as the true 

 variation ; the first needle was then laid on a third, witli t!ie 

 north end of the one in contact with the south end of the other, 

 as by this means the magnetic power is preserved, if not 

 increased. 



The meteorological journal during the last months is not so 

 full as it was my intention to have made it : the blanks in the 

 column entitled " Velocity " were caused by the frequent break- 

 ing of the machines with Avhich the wind was measured. The 

 machine best adapted for this purpose, provided the rate does 

 not exceed 30 miles per hour, is described in the Annals fox 

 October, 1818. The other was constructed like a windmill, 

 and a table of the velocity of the sails is given in the Ainiah for . 

 Aug. 1816 : this also, owing to the vast rapidity with which it 

 moved in a strong wind, met with repeated accidents, which 

 caused me to discontinue that part of the table. 



The only merit 1 claim for these observations is industry, and 

 a scrupulous attention to truth ; and should they be found useful 

 in throwing any light on this branch of natural knowledge, I 

 shall have done no more than my <luty towards that learned 

 Society of which I have the honour of beino- a member. 

 1 remam, my dear Sir, yours, very sincerely, 



M \ UK I^t: .\i rov. 

 V J 



