440 



NA TURE 



[April (), 1874 



tube of the funnel was bent upwards and had an orifice about 

 l-20th of an inch in diameter. Close above this orifice was placed 

 a glass slide, held by springs and bearing a drop of gl)xerine on 

 its loner suriace. The tube and glass slide were protected by a 

 roof and two cheeV -boards, which formed the vane of the weather- 

 cock. Tlie glass slide was so placed that the current of air issu- 

 ing from the narrow orifice of the bent tube, under pressure of 

 tlie wind on the wide m juth of the funnel, impinged on the 

 centre of the drop of glyeerine, and a large proportion of any 

 solid matters carried by the air was caught on the glycerine. 

 After a day or two, according to the weather, the slide was re- 

 moved, a thin disk of glass was placed on the glycerine-drop, 

 and the contents were then examined under tlie microscope, a 

 duplicate slide being left in the ^eroscope for the next observa- 

 tion. 



This instrument depended for its function on the wind. I f 

 there was no wind, there was no current through the tube, and 

 nothing was caught on the glycerine ; but in general there was 

 wind enough, and the captures were ample, often embarrassing 

 by their multitude. Tlie observations were mostly made in the 

 neighbourhood of London, at the Greenwich Observatory. The 

 nature of the captures varied according to the direction and 

 velocity of the wind, the state of the weather, and the season of 

 the year. A north-west wind, blowing over London, brought 

 soot and globules of coal-tar, textile fibres, nondescript dSris, a 

 few vegetaVjle spores, now and then an epithelial scale or two, 

 always a number of half-cooked starch-grains {identified by their 

 reaction with iodine and by traces of concentric lamination giving 

 a black cross with the polariscope), and sometimes microscopic 

 bread-crumbs (half-cooked starch-grains in meshes of gluten). 

 The starch-grains were the most constant capture of all, in all 

 seasons and for all directions of wind. They seem to be very 

 durable. (If I remember rightly, M. Pouchet found starch- 

 grains in all specimens of dust, even the most ancient, obtained 

 from the neighbourhood of human dwellings.) A soutlierly 

 wind, blowing from the country, brought a great variety of 

 vegetable spores and pollen-grains and di'h-is^ with a smaller 

 proportion of matters characteristic of town air. The size and 

 quantity of the captures depended m,ainly on the velocity of tlie 

 \i ind. Once or twice a strong wind swept a living acarus, or an 

 entomostracan, or the shell of a. diatom into the glycerine. In 

 dry windy weather a quantity of siliceous sand was caught, which 

 gave trouble by tilting the disk when in preparation for the 

 microscope, and the larger grains had to be removed with the 

 point of a needle. 



The most interesting variation in the character of the organic 

 captures was that which depended on the season of the year. In 

 January and February scarcely anything was found (besides 

 dibris and inorganic matter) but a few fragments of mycelium of 

 some fungus ; but with the first fine weather in March the 

 glycerine began to yield good returns. Spherical grains of 

 poplar pollen were caught in large numbers, thirty or forty in 

 a single drop, though the nearest poplar tree was a quarter of 

 a mile distant. These were soon followed by the triangular 

 pollen of birch and hazel-trees depending, like poplar, on the 

 agency of the wind for fertilisation. From this time onward, 

 tlirougli spring and summer, a great number and variety of 

 pollen grains were caught. Cryptogamic cells increased in 

 number through the summer, and reached their maximum in 

 the autumn, when brown septate spores and others of various 

 kinds, which my imperfect knowledge did not enable nie to 

 identify, appeared in abundance. If left for some days, they 

 began to germinate. Towards winter their number diminished, 

 the latest being minute dark biown oval spores of some species 

 of agaricus (?). The winter months were comparatively barren. 



I did not find any Bacteria, but there were numbers of exces- 

 sively minute particles, of which I could not tell the nature. 

 Once, after leaving the aeroscope for several days, I found the 

 glycerine swarming with a minute torida which had evidently 

 multiplied in that pabulum. In fact the glycerine was fer- 

 menting. 



Among these facts, the only one which seems to have any 

 bearing on the question of the propagation of infectious diseases 

 is the great prevalence of cryptogamic spores in the .air in 

 autumn, when those diseases are esjiecially rife. 



To avoid fallacy in the results obtained, I used to place two 

 drops of glycerine on the same slide, but only directed the 

 air-current against one of them ; both were examined under the 

 microscope, and the difference credited to the air. By using 

 glycerine that had been boiled with carmine, many of the 



organic captures were made more distinct : the nuclei of epithelial 

 icaies and of many other cells were brightly stained by the 

 carmine. Glycerine had the disadvantage of absorbing moisture 

 in d»mp weatlier and swelling to an inconvenient bulk. At 

 such times I used oil instead, with good effect. My plan of 

 examination was to sweep the whole disk in successive parallel 

 zones, by the aid of mechanical stage-movement, and make 

 record of every organic body that could be recognised. Such 

 as I had not seen before were sketched with pen and ink and 

 coloured chalk in a book devoted to that purpose. 



It will be seen that my observations entire y support Mr 

 Cunningham's, as to the abundant presence of living spores in 

 the air. I was satisfied that this branch of research, in the 

 hands of one thoroughly familiar with these microscopic forms, 

 would lead to results of great interest, and I heartily con- 

 gratulate Mr. Cunningham on the valuable work which he has 

 produced. Hubert Airy 



Animal Locomotion 



Mr. Wallace's last letter seems to call for a word of expla- 

 nation from me. I did not refer to the up stroke of the bird's 

 wing because this was not the point in dispute. But in reply to 

 Mr. Wallace's latest stricture — that I appear "to ignore the 

 great downward reaction, added to gravitation, during every up 

 stroke" — I would say (i) that the downward reaction is not 

 great, ((/) because, as Mr. Wallace his himself observed, of the 

 valvular action of the feathers ; (/>) because of the convex form 

 of the upper surface of the wing ; and {c) in some cases, because 

 the wing is less expanded in tlie up stroke. {2) As to the effect 

 of gravitation, this was already allowed for in determining the 

 resultant motion consequent on the down stroke, and must not be 

 reckoned twice. Just as with an arrow shot from a bow, so with 

 the bird ; the motion consequent on the down stroke lasts 

 long enough for the wings to be raised before it is spent. Mr. 

 Wallace is certainly right in saying that the down stroke should 

 counteract the downward reaction of the up stroke, but this 

 downward reaction being slight cannot require " a highly- 

 inclined upward motion," and what is more, it cannot require 

 that the under surface of the wing should be directed forwards 

 as Dr. Pettigrew asserts. 



Again, I do not say the movement of the wing as a whole is 

 downward and backward, but that the action of its surface is in 

 that direction. The Duke of Argyll is no doubt correct in main- 

 taining that the wing as a whole moves in a perpendicular line, 

 or perhaps with a slight forward overlap. 



I cordially agree with Mr. Wallace that the matter is no'; to 

 be settled by "discussing theoretically, but by observation and 

 experiment ; " still the elementary principles of mechanics may 

 surely be heard in evidence without disadvantage even at the out- 

 set of the inquiry. JAMES WARD 



Trinity College, Cambridge, March 30 



Rudimentary Organs 



In a former communication (NATtJRE, vol. ix. p. 361) 

 I promised to advance what seems to me a probable cause — 

 additional to those already known — of the reduction of useless 

 structures. As before stated, it was suggested to me by the 

 penetrating theory proposed by Mr. Darwin (Nature, vol. viii. 

 pp.432 and 505), to which, indeed, it is but a supplement. Epi- 

 tomising Mr. Darwin's conception as to the dwarfing influence of 

 impoverished conditions, progressively reducing the average size 

 of a useless structure, by means of free intercrossing ; the present 

 cause may be defined as the mere cessation of the selective 

 influence from changed condition of life. 



Suppose a structure to have been raised by natural selection 

 from o to average size too, and then to have become wholly 

 useless. The selective influence would now not only be with- 

 drawn, but reversed ; for, through Economy of Growth — under- 

 standing by this term both the direct and the indirect influence 

 of natural selection* — it would rigidlyeliniinate the variations loi, 

 102, 103, &c., and favour the variations 99, gS, 97, &c. For 

 the sake of definition we shall neglect the intluence of Economy 

 acting below 100, and so isolate the effects due to the mere 

 withdrawal of Selection. By the condition of our assumption, all 

 variations above 100 are eliminated, while below 100 indiscrimi- 



* See former commuDication, NATURr, vol. ix. p. 361. 



