578 



NATURE 



\Oct. 16, 1879 



temperature of a place, other things being equal, is proportional 

 to the heat received from the sun. " 



His reviewer in the Quarterly for July last says : " The mean 

 January temperature of England may be taken at 39° F., which 

 is equivalent to 278° F. of absolute temperature" (meaning, 

 above the temperature of space taken at - 239° V .), " and if we 

 calculate what would be the mean temperature of the same month 

 when the sun was distant 97,500,000 instead of 91,000,000 of 

 miles as it is now, we find it comes out 242° F., which is equiva- 

 lent to 3° F. of our thermometer, or 29° of frost." 



If we use the same method to find to what extent the present 

 value of the eccentricity ought, even now, to affect temperatures 

 on the earth's surface, we arrive at a result apparently so contrary 

 to experience that I think "there must be a mistake somewhere." 

 I ask your readers to lell me where. 



Let 5 be the temperature of space. Choose two places in equal 

 north and south latitude ; and let U, U' be their July tempera- 

 tures respectively. A, A', their January temperatures, i.e., at 

 aphelion and perihelion ; e the eccentricity. Then we have, 

 according to the principle used by Mr. Croll and his reviewer— 



^•-f ^' _ / I + A ° 

 S + U \i - e)' 



With the present value of the eccentricity, viz., o'oi68, this 

 gives — 



A' = 0-06956'+ 1-0695 U, 



giving to .y the usually accepted value, — 239° F. 

 A' — i6-6i -\- 1-0695 U. 

 .-. ^'- ?/= i6°-6i +0-0695 ;/ . . . (i) 



That is to say, the January temperature of the place in south 

 latitude, ought to exceed the July temperature of the place in 

 equal north latitude by more than 17° F. 

 In like manner we find the relation between U' and A to be — 



U' - A = - is°-53 - 0-07^ ... (2) 



That is to say, the July temperature of the place in south latitude 

 ought to be more than 16° lower than the January temperature of 

 the place in north latitude. 



Now it may be replied that geographical and meteorological 

 causes may completely mask these differences. The mean (une 

 temperature of the northern hemisphere is known to be hi;;her, 

 instead of lower, than the mean December temperature of the 

 southern hemisphere, and it is considered that this is sufficiently 

 accounted for by the excess of land there. If this explanation 

 be true, the effect of the excess of land must be capable of 

 increasing the mean temperature not only by the number of 

 degrees by which the northern hemisphere exceeds the southern, 

 but by this amount plus 1 7° F. 



Subtracting (2) from (i) — 



(A' - U') - (U- A) = 32°-l4 + 0-07^ + 0-069 ^■ 



This shows that, so long as A' is greater than U' and U greater 

 than A, this difference is greater than 32". That is, the differ- 

 ence between the excess of summer temperature over winter in 

 the southern hemisphere exceeds the like excess in the fame 

 latitude north by more than 32°. Is there any indication of an 

 excess of annual variation in anything like this extent in the 

 southern hemisphere ? 



But observe the result at the equator. If the latitudes of the 

 two places are continually diminished they will eventually be 

 found both of them on the equator ; in which case yl' and A 

 become identical, and likewise U' and U. Now the right-hand 

 side of the equation being positive, the left-hand side must be so 

 too. Hence -{U-A), which was negative, in becoming 

 A' - U', which is positive, must pass through zero. This shows 

 that one effect of the eccentricity is that it is not under the equator 

 that the January and July temperatures are the same, but under 

 some latitude north of the equator. 



When the two places are both on the equator, or rather when 

 only one place upon the equator is considered, 



A - (J ~ 16-07 + 0-03 (^ + U) (nearly). 



If we put for ^(A + U) the mean temperature of the equator, 

 or 80° F., this equation gives A - U = 21° F. nearly. 



That is to say, the January temperature of a place on the 

 equator ought at the present time to be about 21° F. higher than 

 the July temperature, if the temperafare of space is so low as 

 - 239° F. 



The temperatures themselves would be — 

 ^ = 90' i, U= 69" i. 



1 would ask, therefore, whether there is any indication of so 

 great a difference as the above at any station on or close to the 

 equator. 



MA- Vis not so great as 2i° F., it must be owing to causes 

 whicli diminish A or increase [/. The place being on the 

 equator, would not be reached by tlie north-east trade-winds ; 

 moreover, in July their extension towards the equator would be 

 least. Consequently, they would have little effect to increase (J 

 by bringing warmth from the heated continents. In a similar 

 way the south-east trades would be at their weakest in Jaiiuar) , 

 and have their least effect to diminish A by bringing cold air and 

 water from the Southern Ocean. Meteorological causes would, 

 therefore, seem to tend rather to exaggerate than to mask the 

 difference in question, if the observations were taken in an 

 insular position near the equator. 



I l)elieve there is admitted to be some uncertainty about the 

 value used for the temperature of space. Herschel's investiga- 

 tion in his meteorology may not be thought satisfactory. Hut it 

 is remarkable that Pouillet, follow ing quite a different method, 

 arrived at almost the same result. At any rate the temperature 

 which the earth would assume, were the sun extingui^hed, must 

 be very low. But is it so low as - 239° F. ? If it were, it 

 appears that, if the principle used be correct, those results would 

 follow which I have suggested ; and I ask whether any observa- 

 tions bear upon the question ? It is obvious that it touches Dr. 

 CroU's celebrated theory somewhat closely. O. F'isher 



Harlton, Cambridge, October 4 



Does Sargassum Vegetate in the Open Sea? 



Having had many opportunities of observing patches of 

 "living Sargassum in the open sea" from the deck of H.M.!r . 

 C^allen^er during her cruise in the North Atlantic in the early 

 jjart of the year 1873, 1 venture to offer a few remarks in reply to 

 the above inquiry of your correspondent in Nature, vol. xx. p. 

 552. The track of our ship between Madeira, the Canary Island -, 

 St. Thomas in the West Indies, Bermudas, and the Azores is 

 almost equivalent, as a glance at the map will show, to a 

 complete circumnavigation of the central part of the North 

 Atlantic generally known as the Sargasso Sea. During this 

 cruise Sargassum baccijerum was met with frequently so as lo 

 render the appearance of this seaweed a sight quite familiar 10 

 all on board the Challenger, It was first seen on March 2 in 

 about lat. 22° 30' N., long. 42° W., halfway between the Canaries 

 and the West Indies. Again on March 6, lat. 21° N., long. 49° 

 W., quantities of gulf-weed drifted past the ship. On more thsn 

 one occasion large patches of Sargassum were observed extendirg 

 from the vicinity of the vessel to a great distance. The gulf-we( d 

 was also encountered between St. Thomas and the Bermud:is 

 group, and was last met by us between the latter islands and the 

 Azores on June 18, lat. 35° N., long. 53° W. 



As regards the exact form and appearance of this interesting 

 alga, I cannot do better than quote from the graphic description 

 given by Sir C. Wyville Thomson in the pages of "Tlie 

 Atlantic," vol. ii. pp. 9, lo : — 



" They (the patches) consist of a single layer of feathery 

 bunches of the weed (Sargassum bacci/erum), not matted but 

 floating nearly free of one another, only sufficiently entangled for 

 the mass to keep together. Each tuft has a central brov<-n 

 thread-like branching stem studded with round air-vesicles on 

 short stalks, most of those near the centre dead, and coated with 

 a beautiful netted white polyzobn. Afler a time vesicles so 

 encrusted break off, and where there is much gulf -weed the sea is 

 studded with these little separate white balls. A short way from 

 the centre, towards the ends of the branches, the serrated -A'illow- 

 like leaves of the plant begin ; at first brown and rigid, but 

 becoming farther on in the branch paler, more delicate, 

 and more active in their vitality. The young fresh leaves 

 and air-vesicles are usually ornamented with the stalked vases 

 of a Campanularia. The general colour of the mass of 

 weed is thus olive in all its shades, but the golden olive 

 of the young and growing branches greatly predominates. 

 The general effect of a number of such fields and patches of 

 weed, in abrupt and yet most harmonious contrast with the lanes 

 of intense indigo which separate them, is very pleasing." ' On 

 p. 339 of the same volume we find the following remark : — 

 " Very few of tlie higher algse Uve even occasionally on the 

 surface of the sea ; the notable exception is the gulf -weed 



' "The Atlantic," by Sir C. Wyville Thomson. (London: Macmillan 

 and Co., 1877.) 



