VOL. LXXI.] PHILOSOPHICAL TRANSACTIONS. 57 



1 l h 43 m , the darkest part is almost arrived at the centre. At 12 h 17 m , the dark 

 spot is with its edge just near the centre. 



May 13. Seven-feet reflector ; power 222. Clock g 5 too fast. At 1 l h 26 m , 

 Mars seems now to be in the same situation he was the 1 1th, at 10 h 8 m . 



May 22, clock 4 s too slow. At 12 h 5 m , the figure of May 1 1th not on the 

 disc ; but some other fainter spots are visible. 



June 6, the clock loses V.Q per day. At 10 h 10 m , the same figure is on the 

 disc of Mars which was there April 8, 1777, at 7 h 30 m . 



June 15, clock I7 m too slow. At g 1 ' 45 m , the same figure is on Mars that 

 was there May g, at U h l m ; but it is more advanced. Mr. H. supposed it to 

 be the same, and in the same situation, as April 17, 1777, at 7 h 50 m . 



June 17, clock 20 s slow. At g" 12 m , the dark spot on Mars is rather more 

 advanced than it was May 11th, at 10 h 18 m . At 10" the spot visibly advanced. 



June ig, clock 22 s too slow by the transit of 2 Scorpii observed this evening. 

 At 8 h 40 m , the figure on the disc of Mars appears now to be as it was April 26, 

 1777. At 1 l h 47 m , the state of the air near the horizon is very unfavourable. 

 With much difficulty it can but just be seen that the figure is not quite so far ad- 

 vanced as it was May 11th, at 1 l h 43 m , but can certainly not be above 2 or 3 

 minutes from it. 



Now to examine the result of the above-mentioned observations : comparing 

 together the 2 following short intervals of the year 1 779, we have, 



From May g d 1 l h O m 45 s to May 1 l d 12 h l6 m 48 s = 2 d l h l6 m 3 s = 2 revol. 

 hence 1 revol. = 24 h 38 m T.5 



A 2d small interval. From May ll d 10 17 m 48s to 13 d ll h 25 m 51 s = 

 2 d 1 8 m 3 s = 2 revol. hence 4 revol. = 24 h 34 m P.5. 



Here we have 2 very short intervals that agree to 4 m , which is more than we 

 could have expected in such short peiiods of time. Comparing together obser- 

 vations that were made at a greater distance, we find, first monthly period, from 

 May 1 l d 10 h 17 m 48 s to June 17 d g h g m 20 s allowing 3 m , because the observation 

 says the spot was rather more advanced, = 36 d 22 h 51 m 32 s = 36 revol. hence 1 

 revol. = 24 h 38 m 5 s .g. 



Second monthly period, from May ll d ll h 42 m 48 s to June ig d 1 l h 50 m 22*, 

 allowing 3 m for the time the spot would have taken to come to the place men- 

 tioned, = 3g d h 7 m 34 s = 38 revol. hence 1 revol. = 24 h 38 m 5 S .4. 



Third monthly period, from May 13 d ll h 25 m 51 s to June )7 d g h 9 m 20 s = 

 34 d 21 h 43 m 2g 8 = 34 revol. hence 1 revol. = 24 h 38 m 20 5 .3. 



This last is perhaps as likely to be near the truth as any, since the same spot 

 was here observed for the 3d time, and therefore its motion become more 

 familiar. 



Here we have 3 longer periods that agree to 15 seconds, which is quite suf- 



vol. xv. I 



