230 



KNOWLEDGE 



[December 1, 1891. 



To the Editor of Knowledge. 



Sib, — At page 192 of the October issue you say: "Every- 

 one is familiar witli the earth-smell of the air after a 

 shower of rain, when the lowered pressure of the baro- 

 meter allows the air which has been forced into the soil to 

 rush back to the surface." Being a very early riser, and 

 fond of working in the garden, one of my daily pleasures 

 at this time of the year — often before daylight — is watering 

 the flowers. The " earth-smell " you speak of is very 

 familiar to me, and all through our hot summer weather 

 (practically rahihss) its pleasant, refreshing odour is one 

 of the satisfactions of my morning work. Let me ask if 

 it is not likely that some chemical process may be the 

 cause of the grateful aroma, for how, in the instance I 

 speak of, could lowered barometric pressure find place as 

 a cause ? If I water in the evening, and the ground is 

 damp when I renew work in the morning, no earth-odour 

 is perceptible. Yom-s obediently, 



Charles F. Hart. 

 The Oaks, Claremont, California, 

 October 16th, 1891. 

 [I do not remember to have noticed the earthy smell 

 except after a shower of rain. No doubt it is due to some 

 chemical change which takes place on the contact of air 

 with freshly wetted earth. — A.C.R.] 



THE UPPEK ATMOSPHERE. 

 To tlte Editor o/' Knowledge. 



Sir, — Your article in Kno\\xedge imder this title makes 

 me wish to ask if it is not possible that the earth in times 

 long past gradually robbed the moon of her atmosphere ? 



Y'ou state that a velocity of 7 miles per second would 

 carry gaseous molecules right away from the earth's upper 

 atmosphere into space. 



What speed would carry such molecules _/Vo;« the moon 

 to the forth .' and is such speed possible or probable ? 



A note in Knowledge on this question would I think be 

 of general interest. Yours respectfullv, 



W. M. 



P.S. — If the moon used to be much nearer the earth, as 

 Prof. Ball has supposed, and if her recession was slow, 

 what efiect would that have on the supposed gaseous-robbing 

 powers of the earth ? 



^A velocity of a little more than 2| miles per second 

 would be necessary to carry a projectile shot vertically 

 upwards from the moon's surface out of the region of the 

 moon's attraction. This is so much greater than the mean 

 velocity of the molecules of our atmosphere at temperatures 

 such as probably exist on the moon, that it seems im- 

 probable that the most swiftly moving gaseous molecules 

 could escape fi-om the region of the moon's attraction. If 

 in the earlier stages of the moon's history the moon and 

 earth ever had a common atmosphere it seems more 

 probable that the moon would have robbed the earth of its 

 atmosphere rather than that the earth should have robbed 

 the moon. For the moon, by reason of its greater surface 

 compared with its mass, must from a veiy early stage have 

 cooled faster than the earth, and the cooler body would, 

 imder such circiunstances, tend to rob the hotter, as the 

 gases would condense upon it while they still formed a 

 lofty atmosphere about the hotter body. \Yhatever was 

 the initial condition, there must have been a great evolu- 

 tion of gases during the formation of the lunar volcanoes. 

 Our own geologic history shows that there has been a con- 

 tinual evolution and absoiistion of gases by terrestrial rocks, 

 and probably sunilar changes have taken place upon the 

 moon.— A. C. Ranyard.] 



DARK STRUCTURES IN THE MILKY WAY. 



By A. C. Eanyard. 



THE first plate which illustrates this paper has been 

 made from a photograph taken by Dr. llax \Yolf, 

 of Heidelberg, with an exposure of eleven hours 

 seven minutes made on three different nights, viz., 

 the 11th, l-2th, and 13th September, 1891. 

 It represents a region of the Jlilky ^Yay in the constel- 

 lation Cygnus on the following, or eastern side of the region 

 represented in the large plate published in the October 

 number of Knowledge. The nebulous star x Cygni, which 

 was near to the centre of the plate in the October number, 

 is here seen near to the edge of the plate on the right 

 hand or "preceding" side. Vertically above a Cygni the 

 dark branching structure, an outline of which is given in 

 Fig. 1, will easily be recognized. The reader may assure 

 himself that this tree-hke structure is 

 _^^ # not due to a photographic defect by 



^teif^^^. comparing the plate in the October 

 number with this plate, as well as with 

 the right-hand picture on our second 

 plate, which is a photograph on a 

 much smaller scale of the same region, 

 but includes a greater area of the heavens. 

 This latter photograph was taken by Dr. 

 Max Wolf in a small camera fixed 

 upon the top of the large camera, and 

 driven by the same clock motion. The 

 two cameras were rigidly attached to a 

 telescope, which enabled the driving 

 of the clock to be controlled and the 

 cameras to be accurately directed to the 

 same region of the heavens from night 

 to night. The accuracy with which 

 this has been done is attested by the 

 photographs obtained ; even the smallest 

 stars shown are represented by minute 

 white patches, which appear approxi- 

 mately circular, though our plates are 

 enlarged more than two diameters from 

 the original negatives. 



Our first plate, representing the region 

 about ^ Cygni, was taken with the oy 

 inch Kranz aplanatic camera, described 

 in the October number, and the right-hand picture on our 

 second plate was taken with a smaller camera fitted with a 

 portrait lens of only .55 millimetres (about 2^ inches) aper- 

 ture and 195 millimetres (about 7'8 inches) focal length. 

 The photograph taken with this small camera will bear 

 examination with a lens, and a comparison of it with our 

 first plate and the plate published in the October number 

 is most instructive. The tree-like structure represented in 

 Fig. 1 will be distinctly recognized in all three photographs 

 as darker than the surrounding area. It evidently corre- 

 sponds to a branching stream of matter which cuts out the 

 light of a nebulous background on which it is seen projected, 

 and it is evidently intimately associated with the lines of 

 stars which border the stream and its branches on either 

 side. In addition to the large stars which lie along the 

 edge of the dark area, lines of very minute stars may be 

 traced within the dark structure — they seem to fall into 

 lines conformable to the main stream and branches. A 

 somewhat similar dark branching stream may also be 

 traced on the photograph of the e Cygni region in the 

 October number. One of its branches runs close to e Cygni, 

 and passing northward again branches. The structure 

 represented in Fig. 1 seems to be connected at its base 



Fig. 1. -Diagram 

 showing small 

 dark tree-like 

 structure to 

 the north of 

 a Cvgni. 



