36 On the Solar Eclipse, 



the angular distance of the mountain and this diameter is in- 

 creased, the former will occupy a longer time in passing over 

 any particular point, and a proportionally greater quantity 

 than the real altitude, consequently, obtained. The angular 

 position may be ascertained with sufficient accuracy by the eye, 

 especially if assisted by cross wires in the focus of the tele- 

 scope, and the proper corrections accordingly made. Thus, if 

 CD, (Fig. 3.), the diameter, represent also the direction of the 

 moon's motion, and A the mountain to be measured ; AB being 

 the direction of gravity, Aft is the altitude sought. But instead 

 of moving parallel to A b, the point of contact in the sun's 

 margin appears to move along AE parallel to CD. The moon, 

 therefore, will in reality have travelled in her orbit a space 

 equal to A e, greater than A b, the true altitude as radius : cos. 

 of the mountain's angular position. Hence, when the direction 

 of gravity is perpendicular to that of motion, the altitude of 

 the mountain cannot be ascertained ; for its summit may then 

 move along the margin of the sun's disc, so as not to cross it 

 for many seconds. This consideration obviates an objection, 

 which might have been raised against the accuracy of the 

 above conclusions, from Dr. Halley's observations on the total 

 eclipse of 1715, where he states, that, " For the space of a 

 quarter of a minute, a small piece of the southern horn of the 

 eclipse seemed to be cut off from the rest by a good interval, 

 and appeared like an oblong star, rounded at both ends, in 

 this form. 



which appearance could proceed from no other cause but the 

 inequalities of the moon's surface, there being some elevated 

 parts thereof near the moon's southern pole, by whose interposi- 

 tion that exceedingly fine filament of light was intercepted*." 

 Provided, however, the direction of the perpendicular forms a 



* Pbil. Trans, vol. xxix, p. 248. 



