4 REPORT—1868. 
cular objects for many lunations, so that a normal character of the appearance 
of a certain spot with any given mean solar altitude may be determined. 
Ten observations for each interval would be necessary before the normal 
character could be considered sufficiently ascertained, in order to distinguish 
between apparent and real change. This process is clearly a work of time ; 
it is nevertheless absolutely necessary to settle such a question as that which 
has been raised in respect of Linné. The observations of this spot are now 
numerous ; various opinions have been expressed with regard to it; but as 
yet they are inconclusive, the observations being little better than “raw 
material.” When they have undergone the examination above proposed, we 
may be the better able to arrive at some conclusion respecting Linné. 
In the course of ‘ Zone Observations,” and therefore entirely originating 
with the labours of this Committee, another spot of apparently the same 
nature as Linné, so far as recent observations are concerned, has been detected. 
Not previously known, it has been described under the symbols IV A*™, 
IV AS * as a “bright spot.” The Rev. W. O. Williams of Pwllheli, in 
whose pair of subzones it occurs (see letterpress IV A*, IV AS, pp. 5 & 6, 
and Report British Association, 1866, pp. 241, 242), has fully confirmed an 
observation which I made in 1867 on May 11, when I sawit as a shallow 
crater. Mr. Williams has observed it as a crater with a central cone, Mr. 
Baxendell has seen it as a well-marked shallow crater, and Mr. Williams has 
frequently seen it as a bright white spot, I have lately ascertained, by the 
aid of the Crossley Equatorial, that it is situated on the summit of a mountain- 
range, and is opened in an irregular depression on this summit. For 
a digest of the existing observations arranged in order of solar altitudes 
see p. 29. They are, however, too few in number to determine at present 
the normal character for each group of 6° of altitude ; but there are a few 
differences of appearance with similar altitudes which claim attention. 
Probably the only circumstance that we are acquainted with as connected 
with our own atmosphere capable of rendering an object on the moon’s sur- 
face indistinct, and sometimes obliterating it altogether, is the agitation pro- 
duced by the mixture of air of different densities. Every observer knows the 
difference between good and bad definition, which passes through a variety 
of gradations from the greatest steadiness to the most violent “ boiling.” 
When the normal character of a spot under a certain altitude of the sun is 
well known, departures from this character become apparent; andif the ob- 
servations have been well recorded and all known circumstances capable of 
affecting it registered, these departures stand out as residual phenomena 
awaiting explanation. It may be that the differences above alluded to are 
of this nature; but it is manifestly premature to discuss them until the ob- 
servations have sufficiently accumulated to determine the normal character of 
the spot under every angle of illumination. 
Herr Schmidt has announced that another spot, Alpetragius d [III A» 4], 
has manifested somewhat similar phenomena. I have carefully examined it 
with the Crossley Equatorial, and find it exactly as desricbed by Schmidt, 
viz. a bright nebulous round spot of light, larger than Linné, with a small 
crater on the extreme §. edge of the bright spot. Lohrmann gives a bright 
elongated spot with a small hill on it; and B. & M. have drawn it distinctly 
as a crater. 
The following is the letter addressed by Herr Schmidt to the Secretary of 
the Lunar Committee of the British Association for the Advancement of 
Science. The translation is by W. T. Lynn, Esq., of the Royal Observatory, 
Greenwich :— 
a 
| 
