April, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



8=5 



ASTRONOMICAL. 



Secular aind Seasonal Changes orv the 

 Moorv's Surface. 



For some years past Professor \V. H. Pickering has been 

 engaged in a series of special studies of the lunar surface. 

 The work has been undertaken in connection with the Astro- 

 nomical Observatory of Harvard College, and a full account 

 of the results obtained will be found in volume 53 of the 

 "Annals" of that institution. .A short ycsuntc has recently 

 been presented in Xatiire over the signature of Professor 

 Pickering. 



Briefly summarised, it may be stated that the evidence of 

 structural change on the visible hemisphere of our satellite has 

 been suspected from time to time by various eminent selenogra- 

 phers, notably by Madler, Schmidt, Webb, Elger, and Niesen. 

 In more recent ye.ars the work of Professor Pickering during a 

 long series of special observations in Peru, Jamaica, and 

 California, has led him to the conclusion that physical 

 changes do occur upon the moon, and that they may be 

 classified under three heads, due respectively to (i) volcanic 

 action, (2) formation and melting of hoar-frost, (3) growth and 

 decay of vegetation. 



The first class of phenomena has its most forcible example 

 in the crater Linne, which, according to Lohrmann, Madler, 

 and Schmidt, prior to 1S43, had a diameter of between four 

 and seven miles, whereas its present diameter is three-quarters 

 of a mile. A new crater has also been announced in the 

 vicinity of the well known formation Hyginus. Again, the 

 floor of the crater Plato has repeatedly furnished new forma- 

 tions under telescopic examination at various times. Picker- 

 ing's latest observations show the existence of a crescent- 

 shaped bank, about six miles long by one or two miles broad. 

 Reference to the maps made by A. S. Williams during the 

 period 1879-90, indicates that no object similar to that now 

 existing was to be seen at that time ; but the maps themselves 

 show that slight variations were apparent at different epochs 

 of observation. 



Considering now the second variety of changes, there are 

 numerous tracts on the moon's surface which exhibit varia- 

 tions in light and shade, which from their character lead to 

 the assumption that they are due to the alternate condensa- 

 tion and volatilization of hoar frost. Professor Pickering 

 especially draws attention to the evidences of aqueous erosion 

 on the central peaks of Theophilus and Eratosthenes, and 

 further considers that the strongest evidence for the former 

 presence of water lies in the dried-up river-beds, of which the 

 best example is to be seen on the eastern slopes of Mount 

 Hadley, at the base of the Appenines. Another, discovered 

 only in the summer of 1904, lies about 60 miles due south of 

 Conon. Still more positive, however, is the evidence the 

 author gives of personal observations of the periodical changes 

 in the craters Messier and Messier A, according to the pro- 

 gress of the lunar day — changes which he asserts inay be 

 followed with a telescope, of 4 inches aperture under good 

 atmospheric conditions. 



The third class of phenomena, possibly due to the growth 

 and decay of vegetation, are stated to be more conspicuous 

 than the effects produced by the two former causes. Repro- 

 ductions are given of photographs obtained by Professor 

 Pickering in the island of Jamaica in 1901, showing undoubted 

 evidence of changes in the crater Eratosthenes, progressing 

 very regularly with the age of the moon. Every precaution 

 has been taken to exclude effects due to varying angles of 

 lighting, and it remains quite certain that growths of dusky 

 matter have taken place. As we have no knowledge of any 

 mineral matter which could produce this appearance, it seems 

 proper to ascribe it to vegetable growth. 



Law of the Sun's Rota^tion. 



Professor N. C. Duner, of Upsala, has within the last 

 few years been engaged in a series of observations for the 

 determination of the Sun's rotational velocity at different 

 heliographic latitudes. The observations were made by deter- 

 mining the linear velocity at various points on the Smi'slimb, 

 by measures of the displacement of spectrum lines in the line 

 of sight. In the latest list given he brings together results 

 obtained from various determinations during the period 1887- 

 igoi. The following is a summary of the values found, taking 

 zones of latitude of 15°. 



Ne>v Determination of Solar Constant. 



Monsieur A. Hansky has recently published an interesting 

 account of a series of determinations of the solar constant by 

 actinometric observations on the summit of Mont Blanc, 

 .^s given by various observers, this important quantity has 

 been assigned many values, of which the following are the 

 more important : — 



Pouillet .. .. .. .. .. .. 1-763 



VioUe (Mt. Blanc) 2-54 



Crova (Mt. Ventoux) 2-83 



Langley (Mt. Whitney) 3068 



Savelieff .. .. .. .. .. .. 347 



Angstrom (Peak of Teneriffe). . .. .. 400 



The two latter values are obviously too high in view of the 

 more recent refined determinations. Later determinations 

 with the bolometer by Langley gave the revised value of 2-54 

 as the more probable value. 



M. Hansky's observations were made on the Mont Blanc 

 station during the years 1897,1898, and 1900, giving the mean 

 value of 3-29 as his final result. The series of igoo were made 

 under specially favourable conditions, and full details are 

 given of that section. During twelve days' sojourn there he 

 secured five actinometric curves, and numerous direct deter- 

 minations of the solar thermal radiation, all of the observations 

 being obtained with Crova's actinometer. From July 23 to 28 

 the weather was extremely fine, but the temperature high. 

 The air was almost quite calm, but the aerial currents from 

 the valley were evidently strong, as indicated by the formation 

 of cumuli above the summit of the mountain. The hygro- 

 metric state at the summit was 70 per cent., indicating a 

 vapour tension of about 1-2 mm. 



The polarisation of the sky was about 0-50, and never ex- 

 ceeded 0-67 ; the colour of the sky was normal blue ; baro- 

 metric pressure varied from 426 to 430 mm. 



The actinometric curves were very regular during the morn- 

 ing and evening hours, but showed strong depressions a little 

 before and a little after midday (from 9 h. — 1 h.), exactly as 

 Crova had already found at Montpellier and Mont Ventoux. 

 This depression may in part be attributed to the ascend- 

 ing air currents from the valley carrying with them large 

 quantities of aqueous vapour, thus exerting considerable ab- 

 sorption of the calorific rays. 



* * * 



Seventh Satellite of Jupiter. 



A telegram received from the Kiel Centralstelleon February 

 28, too late for inclusion in our last issue, announced the dis- 

 covery of yet another satellite of the planet Jupiter, presum- 

 ably by Professor Perrine, although the authorship was not 

 mentioned. 



The new satellite was stated to be of the i6th stellar mag- 

 nitude, and its position when measured was 



Position angle from Jupiter = 62° )_ d. 



Distance = 21' ' 1905, February, 25-6 



