1871.] Eclipse at Oxford, December 22, 1870. 291 



It is found by these observations that, 



The sun's diameter being taken at 530 



That of the moon is 540 



The length of the sagitta m s is 100 



Fig. 1. A A, B B, C C, D D, arc lines joining the dots marking the cusps at four suc- 



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 ; 

 m s the sagitta at the moment of greatest obscuration. The moon's path 

 passes below A", above B", and nearly coincides with O" and D", which are 

 the places of the moon's centre for the cusps A', B', 0', D'. 



These numbers, according to the proportions given in the Nautical 

 Almanac for the Radcliffe Observatory, would have been : — 



The agreement is quite close enough to justify the belief that, in skilful 

 hands, the method described may be in some cases very useful, it being by 

 no means limited to eclipses. It is so simple that one can hardly suppose 

 it not to have been already employed ; but I have met with no notice of 

 such being the case. 



During the progress of the eclipse three thermometers were observed' 

 One north of the house, screened from the sun and sky, sank from 20° 

 at ll h 40 m to 24°-4 at l h 2o m . One south of the house, indirectly in- 

 fluenced by solar radiation on neighbouring objects, rose from 26°- 75 at 

 1 l h to 27 0, 8 at noon, then sank to 26° at 12 h 40 m , and rose to 27°"3 at l h 35 m . 

 A third, on grass open to the sky, sank from 2/ 0, 8 at 1 l h 40 m to 23°*5 at 

 l h 25™, and remained at this point till l h 35 m . Though on a limited scale, 

 the influence both of solar and sky radiation is traceable in these observa- 

 tions. 



cessive epochs during the eclipse. 



Diameter of the sun . 

 Diameter of the moon 

 Length of sagitta 



530-0 

 538-8 

 99-1 



