88 REPORT — 1871. 



Bessel were very indistinctly seen, the renifiinder invisible. Posidonius, 

 just -within the terminator, "was fairly defined. SuJpicius Gallus and one 

 or two near it on the pleateau were clear ; so that the moke an object was 

 RAISED above the general level of the Mare the clearer was its deiinition, 

 while those on the level of it were more or less obscured. 



" The 3Iare Frigoris was very hazy indeed ; even close to the foot of the 

 north slope of Pluto objects could not be defined, while those raised a little 

 above the Mare were remarkably well defined indeed. The whole northern 

 slope of Plato appeared everywhere rugged and iineven." 



Indications of intermittent Visibility and of possible volcanic Activity. 



On the evening of the 13th of May, 1870, no less than tiventy-seven spots 

 were seen on the floor of Plato, 26 by Mr. Pratt, and an extra one by Mr. 

 Elger. This extraordinary display occurred between 132 and 144 hours 

 after the terminator had passed 4° E. long. It is, however, not a little 

 remarkable that, on the same evening, Mr. Gledhil!, at Halifax, observed 

 four spots only. The great number seen by Mr. Pratt, as compared with 

 the small niimber seen by Mr. GledhiU, is doubtless due to a fine state of 

 the earth's atmosphere at Brighton. 



With regard to the streaks seen by Mr. Pratt on the same evening he 

 remarks — " I could not see the small streaks on the western part of the floor, 

 and sometimes even my old 'trident'. and the streak c were so indistinct as to 

 be difficult. What was the cause ? Surely not the earth's atmosphere ; 

 for at the same time spots could be seen. Perhaps we shall discover that 

 spots are raised at a higher level than light streaks, and thus visible when 

 streaks are obscured." 



This remark of Mr. Pratt's is important : certainly the state of the earth's 

 atmosphere could not have affected the two classes of objects in different 

 ways. If the intensity of the spots depended upon the purity of our atmo- 

 sphere, one would think that the brightness of the streaks would also have 

 been increased ; but in Mr. Pratt's experience it was not so. Mr. Elger 

 speaks of some as bright and others faint. Mr. Gledhill, with a bad atmo- 

 sphere, speaks of them as bright ; but he saw only four spots. Are the spots 

 really brighter than the streaks ? But, then, why do both vary in brightness ? 



Mr. Pratt having perused [carefully] the MS. has furnished me with the 

 following remarks : — 



" May it not be well to mention that, on the occasion referred to, 1870, 

 May 13, I observed fifteen st reals, one of which was a new one. [This 

 was the streak from spot jS^o. 5 towards No. 14.] This number was much 

 above the average, the cm-ious fact being that although so many were per- 

 ceptible with attention, yet the increase in their brightness was in a lower 

 ratio than that of the spots. There are two possibilities which may aflfect the 

 discrepancy[diflerence]betw'een the notes of Mr. Gledhill and myself in relation 

 to thestreaks: — Eirst, the times at which ive observed, may have been different. As 

 for myself, I tested the chance of working with any thing like satisfaction once 

 at least every half hour during the whole of the evening, and before I tried for 

 the last time, at 1 1 hours, had been unable to perceive either one spot or streak. 

 Secondly, jjriority of observation bestowed on objects of one class may detract 

 from the estimated brilliancy of the other class. In my own case, immediately 

 I went to the telescope, at 11 hours, I saw several spots conspicuously, and in 

 consequence searched for spots alone for nearly an hour. A search for so long a 

 time for one class possibly may, in a slight measure, reduce the sensibility of 

 the eye for objects of the other class, whether spots or streaks." 



The following extracts from Mr. Pratt's letter, dated 1870, May 19, are 



