A7tOHst 23, 1877J 



NA TURE 



341 



every 200 metres of height of the mountains. The heat thus 

 gained is liberated in the condensation of the vapour. I believe 

 this is satisfactory as regards the Foehti. 



But this will not account for a rise of the temperature of 

 Southern Greenland from its mean December temperature, 

 which, according to Dove's map, is below freezing, to 14" C. 

 A rise of 14° C. would require, according to the above law, a 

 mountain chain 2,Soo metres, or about 8,000 feet in height, and 

 there is none in Greenland approaching this. 



I used to think that great rises of temperature in the Arctic 

 ■winter were due to the wind tearing up the Irozen surface of the 

 sea, and liberating the heat of the water below ; but ihis will not 

 account for an increase of temperature above freezing. I have 

 no explanation to offer. Joseph John Murphy 



Old Forge, Dunmurry, Co. Antrim, August 13 



Does Sunshine Extinguish Fire? 



It is a popular belief that a fire will not burn if exposed to 

 the sun, and, from all I have observed, it seems well founded. 

 Can any of your readers favour me with an explanation of the 

 phenomenon, if true ; or is it a mere superstition ? 



Schwarzwald, August 11 Charles Watson 



OUR ASTRONOMICAL COLUMN 



Thk Opposition of Mars, 1877. — The present oppo- 

 sition of the planet Mars oiTers conditions so nearly 

 analogous to those of the opposition in 1S62, that the 

 many fine drawings made in that year, a number of 

 which are contained in vol. .\xxii. of the '' Memoirs " of 

 the Royal Astronomical Society, become available for 

 comparison with such as may be made during the actual 

 opposition. The same hemisphere of the planet is pre- 

 sented to the earth, and our depression from the martial 

 equator is sensibly the same ; thus, at opposition in 1862 

 the angle of position of the visible (southern) pole was 

 I45°'3 ^"'i '■'c earth's depression -22°7, while at the 

 opposition of 1877 the figures are respectively i6o°'3 and 

 - 22° 5. The least distance of Mars from the earth in 

 1862 was o'4o6, while in 1877 it is 0377. 



Notwithstanding Secchi observing in 1858 found the 

 features upon the disc of Alars irreconcilable with those 

 delineated in Madler's drawings made under similar cir- 

 cumstances in 1830, it was sufficiently evident at the 

 opposition of 1862 that these differences are to be attri- 

 buted to the temporary conditions occasioned by clouds 

 of varying density, form, and e.xtent, in the atmosphere 

 of the planet itself, heightened perhaps in some degree 

 by the state of our own atmosphere at the times of the 

 observations. A striking instance in support of this 

 conclusion was afforded by Mr. Lockyer's observations 

 on September 25, 1862. At loh. 44m., when his drawing 

 No. 14 was completed, the well-known spot a of Madler 

 was quite invisible, while when No. 15 was made shortly 

 afterwards, this spot was " among the most prominent 

 features upon the planet's disc." 



, There would appear now to be little doubt that the 

 green and red portions of the disc do really represent 

 seas and continents, and are not due to the effect of cor.- 

 trast, another explanation which has been suggested. 

 During the actual favourable appearance of the planet, we 

 may expect that measures will be made which will admit 

 of a closer determination of the position of the axis of 

 rotation than any yet obtained. The results at present 

 upon record are (i) Sir W. Herschel's, which assigns for 

 the longitude of the ascending node of the equator of 

 Mars upon his orbit, 79° 27' for 1872, and for the obliquity 

 of his ecliptic 28'' 42'. The reduction by Oudemann's of 

 Herschel's measures, make these figures 79'^ iS' and 

 20° S3' respectively ; (2) Schroetei's, as given by M. Tcrby, 

 which places the south pole in 172^ 5+''7, with latitude 

 60^ 33''2, whence we find for 1798, longitude of ascending 

 node of equator on orbit, 84^ 54', obliquity of ecliptic 

 27° 57' for 1798 ; and (3) Oudemann's reduction of Bessel's 



measures with the Kdnigsberg heliometer, made September 

 28, 1830, January 21, 1835, and February 11, 1837, giving 

 for the place of the ascending node 80° 50', and for the 

 obliquity 27° 17' for 1S34. With the Ust values which 

 have been generally adopted, we have for the ascending 

 node of the equator of Mars upon the earth's equator 

 (N), and its inchnation thereto (I) : — 



N = 47 42 ■^ 0-50 (/ 

 I = 39 52 - °'25 U 



1850) 

 1850). 



The following table showing the angle of position of 

 the visible pole of Mars, and the elevation of the earth 

 above the plane of his equator, at the oppositions between 

 1850 and 1880, has been calculated from the above 

 elements, and may be of interest to some readers ; the 

 least distance of Mars from the earth is added : — 



A glance at this table exhibits a well-known condition 

 that when Mars is nearest to the earth and when we have 

 consequently the best opportunities of studying the 

 features upon his disc, his southern hemisphere is always 

 directed to the earth, and hence we are likely to be better 

 acquainted with that hemisphere than with the northern 

 one, which is turned towards the earth only at the greater 

 distances of Mars. 



The Satellites of Saturn. — A series of observa- 

 tions of all the eight satellites of Saturn by Prof. Asaph 

 Hall, dated from Washington in December, 1876, has at 

 last made its appearance in No. 2,145 of the Astrouomischc 

 Nachrichten. 



Satellites of Mars. — A telegram from the Smith- 

 sonian Institution to M. Leverrier^ received August 19, 

 notifies the extraordinary discovery of two satellites of 

 Mars by Prof. Asaph Hall, of the U.S. Naval Observatory 

 at Washington. The telegram runs thus : " Two satel- 

 lites of Mars by Hall at Washington, first elongation 

 west, August 18, eleven hours, Washington distance, 

 eighty seconds, period thirty hours, distance of second, 

 fifty seconds." 



THE BRITISH ASSOCIATION 



Plymouth, Tuesday 



THE Plymouth Meeting of the British Association has 

 not realised, as far as numbers are concerned, the 

 success which we anticipated last week, and which was 

 indicated by the business done on the days immediately 

 preceding the opening of the meeting. 



The attendance of the regular members from a 

 distance has been very good indeed, and can compare 

 favourably with meetings that have gone before it, 

 but the visit of the British Association, while opening 

 wide the gates of hospitality of the people of Ply- 

 mouth, does not seem to have awakened the scientific 

 interest of the community sufficiently to cause many 

 to enlist in its ranks. It is seldom that so small a 

 niimber of local members have been added to the list 



