290 



Prof. J. Phillips's Observations of the 



[Feb. 9, 



fluence of muscular effort upon tliem, and perhaps for the earlier or later 

 sense of fatigue. The sphygmographic tracings added by M. Lortet to his 

 temperature-charts seemed to show a great inadequacy of circulation. 



II. "Observations of the Eclipse at Oxford, December 22, 1870." 

 By John Phillips, M.A., D.C.L., F.R.S., Professor of Geology 

 in the University of Oxford. Received December 28, 1870. 



At my observatory, situated about one third of a mile eastward from the 

 great establishment founded in the name of Dr. Radcliffe, the 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 intervals 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 convenient for 

 other reasons sometimes to shift the image vertically ; the method which I 

 employed, however, was independent of these displacements, 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, C C, D D. 



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. 



The line of motion of the moon's centre is obtained by ruling through 

 the mid points between A" and B", B" and C", C" and D". The point 

 on this line reached by the moon's centre at the moment of greatest ob- 

 scuration is found by bisection in M. Drawing through M and S the bi- 

 secting line of greatest obscuration, the length of the sagitta m s is deter- 

 mined. 



