382 



NATURE 



[Mar. 19, 1874 



directed downwards and backwards must give an impetus upwards 

 and forwards ; tlie s(il/ mrface so directed will glide downwards 

 and forwards. 1 do not deny that if the down stroke of the 

 bird's wing be directed backwards, beyond a certain angle, the 

 resultant motion will be, as Mr. Wallace says, " obliquely 

 downwards." But why? Because all the sustaining forces above 

 enumerated are so seriously diminished — the horizontal and 

 forward forces, with the exception of (rf), biin^ increased — that, 

 to use Mr. Wallace's words, " the surplus vertical reaction of 

 •he down stroke over the up stroke is no longer able to over- 

 come gravity," which converts the bird's wings for the nonce into 

 kites as it comes sailing downwards, making but an occasional 

 strike, now that the horizontal effect of the wings is so great, to 

 increase the obliquity of its descent. 



But within the limits of this angle, whatever they be, the effect 

 of a downward and backward blow must, on mechanical grounds, 

 be in general such as I hive said. For clearness' sake it may 

 not be superfluous to note an ambiguity in the expressions 

 "downwards and backwards," '' downwards and forwards;" 

 they may apply either to the direction of tile surface of ths wing 

 or to the direction of the anterior margin. I maintain only that 

 the direction of the surface — in some wings, merely that of the 

 anterior portion of the surface — is downwards and backward;. 

 The anterior margin, by the contraction of the great pectoral 

 muscle, is drawn downwards and forwards, in which, by the way, 

 there is the further advantage that less air will escape from under 

 the wing in front. 



Bu', secondly, what can we observe as to the down stroke? 

 (i) A fact, pointed out to me by an anatomical friend — that the 

 great pectoral muscle which depresses the wing is inserted into 

 a crest situated on the upper and forward side of the head of the 

 humerus, so as to tilt the under surface of the wing slightly up- 

 wards, i.e. eive it a backward direction. (2) If the flight of 

 rooks, or still better of pigeons, be watched from a window to- 

 wards sunset, the position of the shadows on the under side of 

 the wings will be found pretty conclusive as to their direction. 

 (3) The forward inclination of the wings of a bird about to alight, 

 which shows that the motion of the wings in such a position re- 

 tards flight. (4) The action of heavy land or water birds, that 

 have to attain some momentum by the use of both feet and wings 

 before lliey can rise ; here surely a forward blow against the air is 

 manifestly absurd. (5) "The highly-inclined position of a 

 hovering bird," noticed by Mr. Wallace, and not of the bird 

 only, but of his wings. 



Mr. Wallace's closmg remark is both true and sound : — "A 

 bird's wing is a highly complex apparatus, subject to a variety of 

 flexures and motions in every feather." Still it is possible, even 

 probable, that all this variety is referrible to a few simple prin- 

 ciples. It is with these alone that I have ventured to concern 

 myself. James Ward 



Trin. Coll., Camb., March 3 



With reference to Dr. Pettigrew's letter in Nature, vol. ix^ 

 p. 362, I cannot do better than ask him to read the two papers 

 that I refer to in my former reply, which he has evidently not 

 done. 



March 16 A. H. Garrod 



The Moon's Want of Atmosphere 



Your very suggestive review of Messrs. Nasmyth and Car- 

 penter's work on " The Moon " leads me to propose an explana- 

 tion of the absence of a lunar atmosphere, which I do not 

 remember to have seen anywhere. The many arguments in 

 favour of the temperature of the lunar surface being near or at 

 the alisolitte zero, when added to the equally probable sup- 

 position that at the absolute zero all matter assumes the solid 

 form, makes nothing more probable in my mind than that it is 

 the consolidation from cold of all the previously existing gases 

 and vapours of the Moon which has caused its atmosphere to 

 disappear. I'rof Frankland's theory of the frozen condition of 

 the lunar surface is evidently diiferent from the above, and Lord 

 Rosse's obseivatiors on lunar radiation apply only to the direct 

 reflection of the solar rays. A. 11. Garrod 



On Volcanic Eruptions 

 A passage in Nasmyth's work on the " Moon " suggests, as 

 a consequence, an explanation of volcanic eruptions that I have 

 often given in lectures. The point to be explained is, why they 

 are sudden and intermittent. Processes of cooling and expan» 

 siou ate gradual. 



I postulate (l) that a solid crust is shrinking as it cools ; (2) 

 that the liquid interior expands on solidifying ; (3) that the 

 melting-point of lava is lowered by pressure. 



Let us start with a volcanic vent in which the aperture has 

 become partially stopped by cooled or solidified lava. In the 

 region below, pressure sets in from the cooling and ultimate 

 solidification of part of the liquid mass. Hence the melting- 

 point of the rest is lowered by (3). The process continues until 

 the pressure becomes sufficient to relieve itself through some 

 vent, old or new ; a lava rises in the vent. But this relieves the 

 pressure, and it follows from {3) that iiicMre rock will solidify, 

 suddenly, and in so doing force liquid rock rapidly up the vent. 



A volcano is, in fact, a geyser of lava. 



I do not remember to have seen this in any book ; and it 

 perhaps would have been hazardous to assert postulate (2) 

 as certainly true previous to Nasmyth's experiments ; but I have 

 thought it probable : and if it is true, postulate (3) follows, I 

 believe, from the laws of heat, and the explanation will be 

 sound. I shall be glad to hear what is thought of it by autho- 

 rities. 



Rugby, March 13 J. M. Wilson 



Remarks on Ozone 



H.WING perused Dr. Moffatt's interesting communication on 

 Ozone in the Scottish Meteorological Journal of October last, 

 and also noticed the ])aragraph on the subject in the Medical 

 Ti'ncs drawing attention to it, I beg to send the following 

 remarks respecting some points in it open to criticism from out- 

 siders : — • 



1. The numbers, and special jviz« 0/" rcfc^f/j, are not stated 

 in the statistical tables, which might be of importance for com- 

 parison with other persons' records. , 



2. The occurrence of ozone with hail, and not with snow, 

 may be explained by its happening in warmer weather, and not 

 in winter, and in the warmer stratum of air through which the 

 hail falls from the cold stratum above. 



3. The larger quantity of ozone in Table II. in winter over 

 that in summer is anomalou,=, and inconsistent apparently with 

 the records in Tables VI. and VII., where it is stated to increase 

 with the temperature, 



4. If ozone be thought to increase in quantity with incrcast of 

 elevation above the level of the sea, it may be asked how that is 

 to be reconciled with the greater prevalence of it at the sea-shore 

 than inland. 



5. If there be only an apparent connection between electrical 

 storms and ozone, explanation may be required to account for the 

 production of artificial ozone by electrical action, and whether 

 the two be identical in constitution if not in origin. 



6. The paragraph — " The air is drier near the tropics than 

 about the equator," might be more clearly defined by adding the 

 "tropical circles of cancer and Capricorn," as within those lines 

 it certainly gradually gets more and more humid. 



7. In one paragraph there is stated to be an intimate con- 

 nection between huviidity of the atmosphere and the manifestation 

 of ozone, and in another this is stated to be purely accidental, 

 which is ambiguous, while the testimony in support of it is not 

 in accord with that in Tables IX. and X., where the adverse 

 record is apparent. 



S. That the absolute humidity of the air diminishes with in- 

 crease of elevation may be true in the case of lofty balloon 

 ascents, away from any terrestrial influences of mounlains, but, 

 as pointed out in a note, the relative humidity increases, as we 

 may see in Westmoreland or Dartmoor, where the heights are 

 always misty and damp. 



9. The tropical or /;Wi'K';W^ only chance to be land winds 

 in some such regions as the North Indian Ocean, whereas they 

 are generally said to be sea-breezes, as in the South Atlantic 

 Ocean, in the ordinary acceptation of the term. 



10. The connection of the production of ozone by the means 

 of turpentine will bring to the mind of the tourist the freshness 

 of the air of hills planted with pine forests. 



11. Accepting the theory that ozone is connected with the 

 equatorial winds, it may be asked how the increase of ozone in 

 the calm belts is to be accounted for, where there are only 

 Polar winds, converging to ascend into the upper regions of the 

 air from north and south. 



12. The table of obser-vations at sea on board ship would re- 

 quire to be supplemented by a note of the period of the year 

 and number of days on record, as the quantity of ozone is al- 

 ready stated to vary with the seasons of the year (Table II.), and 



