﻿Prof. J. Phillips on the Eclipse at Oxford, Dec. 22, 1870. 397 



beginning of the eclipse was obscured by a passing cloud : the end 

 was recorded at 13 h 38' 38"= l h 35' 0"'9 Oxford mean time. 



The progress of the obscuration was observed at unclouded inter- 

 vals in the first half of the period, continuously during a clear sky 

 in the latter half. Finding it impracticable to observe and measure 

 with ordinary micrometers in the early part of the phenomenon, I 

 arranged to throw the image on a screen, and make my measures on it. 



The driving-clock was affected by the extreme cold, so as to make 

 it difficult to keep the sun's image to one place, and it was conve- 

 nient for other reasons sometimes to shift the image vertically ; the 

 method which I employed, however, was independent of these dis- 

 placements, and allowed of as many measurements of the cusps as 

 might be desired. 



It consisted simply in marking at any moment with pencil the 

 situation of the cusps on the screen, and appending to each dot 

 the time by the sidereal clock. Joining, after the eclipse, these dots 

 by a straight line, and then transferring a parallel line of equal length 

 to meet internally a circle representing the limb of the sun, of the 

 same diameter as the solar image, the chord of the cusps at the given 

 time was obtained, from which, by an easy method, the place of the 

 moon's centre at the moment was derived. The apparent diameters 

 of the sun and moon were obtained by measure of arcs on the screen. 



The diagrams exhibit the whole process. In diagram fig. 1, four 

 of the lines are drawn from the dots on the screen, A A, B B, CC, DD. 



In fig. 2, equal and parallel lines are transferred to the solar circle, 

 whose centre is S, so as to touch it internally at A' A', B' B', C C, 

 D'D'. For each of these lines the centre of the moon's place is 

 marked (A", B", C", D") ; thus the line of the motion of the moon's 

 centre is given, and the phase of greatest obscuration determined. 



Fig. 1. A A, B B, C C, D D, are lines joining the dots marking the cusps at 

 four successive epochs during the eclipse. 

 2. A' A', B' B', C C, D' D', are four lines equal and parallel to A A, 

 B B, C C, D D in fig. 1, and made to touch internally the solar circle, 

 whose centre is S ; ms the sagitta at the moment of greatest obscu- 

 ration. The moon's path passes below A", above B", and nearly 

 coincides with C" and D", which are the places of the moon's centre 

 for the cusps A', B', C, D'. 



