Royal Society. 53 



a lunar day, having two easterly maxima nearly at opposite points 

 of the hour-circle, and two westerly maxima also at nearly two op- 

 posite points of the hour-circle. The extreme elongations are not 

 at precisely opposite points of the hour-circle at any of the three 

 stations, nor have the amounts of the two elongations which take 

 place in the same direction always precisely the same value ; but 

 the slight inequalities in these respects are within the limits which 

 might be ascribed to accidental variations, and might therefore dis- 

 appear v-ith longer continued observations. It is otherwise, how- 

 ever, in the author's opinion, with the disparity between the amounts 

 of easterly and westerly extreme elongations which presents itself at 

 each of the three stations. At Hobarton and St. Helena the westerly 

 elongations liave the larger values, at Toronto the easterly (the north 

 end of the magnet being referred to in all cases). 



The times at which the extreme elongations in the two directions 

 take place are not the same at the three stations, and are as follows: — 

 At Toronto the easterly extremes take place about the hours of 

 and 12, being the hours of the upper and lower culminations; at 

 St. Helena the %vesterly extremes about two hours before the culmi- 

 nations ; and at Hobarton about two hours after the culminations. 

 At Toronto the westerly extremes take place about the hours of 6 

 and 18 ; at St. Helena and Hobarton the easterly extremes respect- 

 ively two hours before and two hours after the same hours of 6 and 

 18.' The extreme inequality, or the amount of lunar variation 

 measured from one extreme elongation to the other, is about 28" of 

 arc at Toronto, 20" at Hobarto^n, and 11" at St. Helena. The 

 resolved portion of the terrestrial magnetic force which acts in the 

 horizontal direction, and is opposed to any disturbing influence, is 

 approximately 3-54 at Toronto, 4-51 at Hobarton, and 5-57 at St. 

 Helena. 



Nov. 24. — The Earl of Rosse, President, in the Chair. 



The following extract of a letter from Lieut. Gilliss, U.S.N, to 

 Colonel Sabine, R.A., was read : — 



Washington, 12 Sept. 1853. 



Lieut. Mackai returned to the United States in April, having made 

 his magnetical obser\'ations successfully at all the elevations and at 

 distances of 100 miles, entirely across the Pampas. Soon after leaving 

 Mendoza he was thrown from his horse, breaking his barometer and 

 so injuring his chronometer, that he has neither the longitudes of 

 his magnetical stations nor barometric profile of the country. Being 

 desirous to make his work com])lete, he volunteered to retrace his 

 ground, and left the United States for the purpose more than a month 

 ago, taking with him the declinometer and dip-circle, two Bunten's 

 barometers, an apparatus for determining altitudes from the boiling- 

 point, and some smaller instruments. 



Conveyance of the unifilar would have required another mule. 



Cursory inspection of the observations already made afforded evi- 

 dence of their reliability, and as repetition would liave involved 



