432 Dr. Lamont on the Ten-year Period which exhibits itself 



From this we ascertain that a maximum took place in the 

 year 1817. 



To obtain further data, we must go back to the observations 

 of Gilpin and Cassini. The former observed from 1786 to 1805 

 in the meeting-room of the Royal Society in London. Two cir- 

 cumstances, however, oppose the use of his results : first, they 

 are incomplete, a period of six or seven months in one year being 

 sometimes omitted ; and secondly, the compass which he made 

 use of does not seem to have been trustworthy. Gilpin, indeed, 

 always repeated his observations, in order to lessen as far as pos- 

 sible the insecurity of his readings. We must still, however, 

 look with some distrust upon results derived from a needle, 

 which, when caused to oscillate, often deviated eight or ten or 

 even a greater number of minutes from its former position. 

 From 1795 to 1805, the solstitial months, June and December, 

 and the equinoctial months, March and September, ai-e given. 

 I have set down these, and also the mean of the four months, in 

 the following table : — 



These numbers do not by any means support the hypothesis 

 above stated ; a decided period is not at all exhibited. 



It is otherwise, however, with the observations of Cassini, 

 which commence two years earlier than those of Gilpin, but are 

 only carried out to 1788. The records were made on the 4th, 

 12th, 20th, and 28th of each month, and give the following 

 results : — 



Magnitude of the 

 diurnal motion. 



Magnitude of the 

 diurnal motion. 



1784. 

 January 

 February 

 March 

 April 



8-77 



8-97 



10-72 



11-16 



1784. 

 May 

 June 

 July 

 August 



Magnitudevof the 

 diurnal motion. 



1784. 



12-22 

 11-44 

 1003 

 10-81 



September 

 October 

 November 

 December 



11-71 

 911 

 6-23 

 4-46 



