Septemberlth,] 820. 37 



sufficient angle with that of motion, there is no cause to prevent 

 the application of the present method to the measurement of 

 mountains in any position. Indeed, from some observations 

 made towards the end of the eclipse, but which a derangement 

 of the pendulum rendered too inaccurate to be detailed, it ap- 

 peared, that even superior advantage would be obtained from 

 observing such mountains as lay considerably removed from 

 the line of motions, as will be seen from inspecting the figure. 

 For, whilst the edge of the solar disc has seemed to move along 

 either of the sides of the mountain, the moon has passed 

 over the respective distances A e and g c. By noting the time, 

 then, in which g c as the largest, is described, and drawing 

 dcB in the direction of the perpendicular, the true altitude 

 may easily be found, while the observer enjoys the same ad- 

 vantage as if the mountain subtended an angle of nearly thrice 

 its real elevation. 



Hitherto the point of contact has been supposed to occur in the 

 circumference of the solar disc, it is plain, however, that when 

 there are spots on the surface, sufficiently conspicuous, a point 

 in their extreme boundary may be employed with greater ease 

 and success, both as respects the obstructions arising from the 

 circular form of the marginal limit, and those occasioned by 

 the difficulty of ascertaining the exact moment of appulse. 

 For during the time of observation, it was found that the dark 

 summit of the mountain mingling with the almost equally dark 

 atmosphere, immediately on egress became invisible ; hence the 

 utmost care and circumspection were frequently inadequate to 

 mark with sufficient precision, the instant of contact, before any 

 part had yet crossed. Had circumstances, on the contrary, 

 permitted observations to be made from the line bounding a 

 solar spot, the matter would have been comparatively easy. 

 The effect of contrast being scarcely impaired, the outline of the 

 mountain would be as apparent when moving over the slighter 

 shade of the spot, as when opposed to the superior brilliancy 

 of the unclouded surface, and hence its progress would be 

 easily traced. This method presents likewise the important ad- 

 vantage of enabling the observer to obtain two measurements 



