450 



NATURE 



[March 22, 1877 



or at most three half-prisms " more dispersion than was before 

 obtained by " ten whole ones," does so at the cost oiall definition, 

 and will be certainly allowed at Greenwich, as well as every- 

 where else, to be a mistaken step in modern spectroscopy before 

 another anniversary of the Royal Astronomical Society takes 

 place. PiAZZi Smyth, 



Edinburgh, March 17 Astronomer-Royal for Scotland 



Greenwich as a Meteorological Observatory 



In Nature (vol. xv. p. 421) there appeared a brief abstract 

 of the presidential address of Mr. H. S, Eaton to the Meteoro- 

 logical Society of London on February 21. The increase of 

 temperature at Greenwich in recent years is stated to be in 

 reality due to local causes and not to secular variation, to which 

 it has, as he thinks, been erroneously assigned. The effect of 

 tlie growth of the population of London from 900,000 at the 

 commencent of the century to 3,500,000 at the present time, 

 and the still greater increase in the comparative consump- 

 tion of coal, Mr. Eaton considers to be manifested by the rise in 

 the average temperature of the air at the Royal Observatory, 

 and for this reason it is concluded that Greenwich is not a suit- 

 able place for a meteorological observatory of the first order. 



If the view enunciated by Mr, Eaton be correct, it is evident 

 that the temperature of Greenwich during recent years has been 

 in excess of that of surrounding districts. Is this view borne out 

 by observation ? Taking the figures for a number of places in 

 the south-east of England whose mean temperatures have been 

 calculated for the same thirteen years ending 1869, and adding 

 the usual correction for height above the sea, we otatain the fol- 

 lowing results as their mean winter, mean summer, and mean 

 annual temperatures ; Greenwich, 40°'4, 63°T, and 5i°'l ; Cam- 

 den Town, London, 40° '4, 63° "3, and 51°' I ; Royston, 40° -5, 

 62°'3, and 50° -8; Colchester, 39°'4, 62°-8, and 5o°-6 ;i\Vortbing, 

 4i°'i, 6i"-2, and 50°7; Osborne, 42°'o, 62°-5, and 5i°'8; 

 Aldershott, 40'9, 62° 6, and 5i°'2 ; and Oxford, 40°'6, 6i°'3, 

 and 50° '4. A simple inspection of these figures is sufficient 

 to show that the consumption of fuel and the vast popu- 

 lation of London cannot be said to have had an appreciable 

 influence on the temperature as recorded at the Royal Ob- 

 servatory, and that if the Greenwich observations show a 

 rise of temperature during recent years, the whole of the 

 south-east of England has shared in that rise. This result 

 deduced from observations is such as might have been expected 

 when the position of the thermometers at Greenwich and the 

 mode of escape of the artificially heated air by chimneys into 

 the free atmosphere is taken into consideration. It follows, 

 therefore, that, so far at least as regards the temperature obser- 

 vations, the conclusion drawn as to the future of our great 

 national Observatory as a contributor to the higher meteoro- 

 logical researches is not supported by the facts of observation. 



Alexander Buciian 



Atmospheric Currents 



I AM glad to have obtained from such exponents as Capt. 

 Digby Murray and Mr. Murphy a clear statement of the old 

 orthodox creed respecting the movements of the atmosphere. 



The former, it is true, finds a difficulty in accepting Maury's 

 belief that the currents cut one another in "curdles" in the 

 equatorial calms, but none in adopting the same as regards 

 the tropical calms, and his view may therefore, as I suppose, be 

 taken as a modification of that which is graphically represented 

 on Plate I. in the " Physical Geography of the Sea." 



The question at issue between Capt. Digby Murray and myself 

 amounts to this : Are rapid polar and equatorial upper currents 

 observed over the region of tropical calms ? Mr. Murphy's theo- 

 retical question appears to me to involve the inquiry — Is the force 

 of the trades derived from the earth's rotation ? 



In tracing the course of the air particles along the route which 

 he describes, the late Commodore Maury observes that this 

 course is determined in certain particulars by " some reason 

 which does not appear to have been very satisfactorily explained 

 by philosophers." The latter do not as yet seem to have got 

 rid of all the difficulties with which his theory is beset, which 

 rather grow with its development. 



I would beg the philosophers to look closely at the actual 

 course of the atmospheric currents as shown by synoptic charts, 

 not by charts of prevailing winds and mean pressures, which 

 represent conditions never found at any one time in nature. The 

 distinction between the " great currents " and the " temporary 

 currents" is important enough, but it amounts to little more 



than that between mean winds and actual winds ; and to explain 

 the mean winds on one principle and the actual winds on the 

 opposite involves a fallacy. 



Again and again we see a more or less irregular belt of high 

 pressures, having central calms, extending across the North 

 Atlantic. From the southern edge of this belt we may follow a 

 particle of air in its course to and from the equatorial district of 

 low pressure, also an irregular belt in the middle of which calms 

 exist. The movement originates in the defect of pressure near 

 the equator at the level occupied by the particle. Its velocity is 

 governed by the steepness of the gradient ; and its direction, in 

 relation to the surfaces traversed, is affected by the increasing 

 velocity of rotation of those surfaces. In the Doldrums it arrives 

 at a district at which the gradient becomes zero and the hori- 

 zontal movement has consequently disappeared ; but a ver- 

 tical movement has now been acquired from the difference in 

 the tension of the particles above and beneath, a difference 

 derived from solar heat. When the particle has ariived at a 

 position in which this difference disappears the vertical movement 

 vanishes, and a new horizontal movement commences owing to 

 the defect of pressures on the polar side at the level then reached, 

 and the direction of this movement is also affected in relation to 

 the surfaces by their decreasing velocity of rotation. Where does 

 the new movement terminate? Obviously in some district be- 

 tween the equator and the pole where horizontal pressures on 

 all sides of the particle at its then level are equable, but where a 

 vertical movement has been acquired, the particles near the 

 earth's surface starting on their journey towards the equator. 

 The onus disptctandiW&s with those who deny that such a district 

 is presented by either of the belts of tropical calms. 



We now look at the polar side of the calm belt of Cancer, and 

 for this purpose we may take almost any, e.g., of Capt. Hoff- 

 meyer's charts. We see in the majority of cases an aggregate 

 of cyclonic circulations around local barometric minima, inter- 

 fering and imperfect, and commonly becoming more so as they 

 are propagated towards the pole. But we see no " polar depres- 

 sion " distinct from these, on which, as represented in the chart, 

 we can lay the finger and say, *' This is the result of centrifugal 

 force; those are due to steam power." Within these systems 

 an upward movement of the air occurs, owing to vapour con- 

 densation and the liberation of heat. Consequently towards 

 these the particles of air near the earth's surface at the poleward 

 edge of the tropical calms begin to travel, the earth's rotation 

 deflecting their course in relation to the surfaces traversed. And 

 from these, at a certain elevation, the particles return to the tro- 

 pical calms for the same reason as that which determines the 

 upper currents over the trades. 



From the phenomena observed in the northern hemisphere I 

 argue, mutatis mutandis, to those of the southern, and I expect 

 the argument to be admitted by one who, like Mr. Murphy, 

 attributes much less influence than I do to the work of water- 

 vapour, and who even thinks that the mean movements of our 

 atmosphere would be unaffected by the removal of all the water 

 of our globe. 



On some occasions pressure is high over all the North Atlantic 

 on the polar side of the tropic, the anticyclones apparently ex- 

 tending nearly to the pole. In these cases we have no surface 

 counter-trades over that district, yet the north-east trades con- 

 tinue to blow on the southward of the tropic as usual, 



I repeat that all movements of the atmosphere originate in 

 differences of pressure derived directly and indirectly from soLir 

 heat, and not in the force of the earth's rotation. And I must 

 add that it seems to me very strange that any one while regarding 

 the trades and their upper-currents as simply the i?^rcA- of pressure 

 differences in the lower latitudes, should maintain that the south- 

 west and north-west winds of the temperate zones are simply the 

 causes of the pressure differences in the higher latitudes. It 

 would be just as logical to regard the south-west and north-west 

 winds as due to pressure distribution, and the trades as the com» 

 pensation for their eastward movement. W. Clement Ley 



March 10 



Electrical Phenomenon 



Last night I noticed a powerful developmfnt of electr 



in a curious manner. I had thrown a piece of common, thick, 

 white, unglazed paper upon a low fire which was tolerably fidl 

 of ashe?. When it was charred so as to be black and brittle, I 

 happened to take it up and break bits off. To my astonishment 

 they stuck firmly to my fingers. I broke off two pieces each an 

 inch long, and resting them on the tips of my two fore-fingers, 



