Solar Eclipse of July 28, 1851. 109 



of the longitude by this means. During the eclipse there 

 was seen a range of elevation measuring in length about 150", 

 and of which the altitude was some 6" above the general out- 

 line. Now the moon's mean motion is roughly one second of 

 arc in two seconds of time, and the occultation of a star by 

 this ridge would therefore have preceded by no less than 12", 

 the computed occultation ; and even by a longer time if 

 the appulse were oblique. A very small change of latitude 

 also will bring the observer opposite to a different part of one 

 of these irregularities. 



This unevenness of outline also opens up a discussion as to 

 what is to be considered as the moon's diameter. With a 

 power of only 50 the bright edge is seen as an irregular line not 

 merely indented and studded with prominences, but irregular 

 in the large way, that is, deviating greatly from the circular 

 form. Photographic portraits of the moon in various states 

 of her libration would be of immense value in enabling astro- 

 nomers to compute exactly an occultation. On our globe 

 the ocean spreads over the greater part of the surface, and, 

 by its unity, determines the general form ; but on the moon 

 there is nothing analogous. Even those spots which appear 

 with a low power as extensive plains, are found, when more 

 narrowly examined, to be undulated and even studded with 

 small volcanic mountains. What then is to be regarded as 

 the moon's diameter, and from what starting point are we to 

 reckon the heights of the lunar hills? 



To return to our eclipse ; the rapid formation of heavy 

 clouds warned us of the propriety of determining our local time, 

 for which purpose the altitude of the sun's lower limb was 

 taken with Adie's circle, face east and face west, and then the 

 end of the eclipse was waited for ; a few seconds before it 

 happened the sun's disc was completely concealed by a cloud, 

 and thus we had neither the beginning nor the end from 

 which to compute our longitude. 



On the evening of the 29th we determined the latitude 

 from the observed altitudes of a Polaris and of Vega to be 

 north 44° 36' 17". The chronometer was also compared at 

 the same time with the local siderial time, and thence, by 

 help of observations made here before our departure and 



