WLtm 



FOR GENERAL READERS. 



FRIDAY, AUGUST 3rd, 1888. 



PAGE 



Scientific Table Talk 97 



The Fourth Expedition of the Hiron- 



delle (?'//«*.) _. 98 



Mechanical Refrigeration (illus.) ... loo 



Bird's Nest or Elephant Island ... 102 



General Notes ... 103 



Improved Laboratory Oven and Bath 



(illus.) 105 



Simple Experiments in Physics (illus.) 105 



The Number of Centenarians... ... 106 



Natural History : 



The Dragon Tree (Mas.) 107 



A New Silkworm .' 108 



An Advocate of Parasitic Vermin 108 



CONTENTS. 



Miscellaneous Notes 

 Our Knowledge of the Bacteria 

 Poisons in the Workshop 



Work for Naturalist Clubs : 



III.— The Structure of a Feather 

 (illus.) _ 



Reviews : 



Pallas's Sand Grouse 



Electricity v. Gas 



The Architect's Register ... 



Abstracts of Papers, Lectures, etc. 

 Royal Society of Edinburgh 

 Society of Engineers 



PAGE 

 108 

 I09 



Royal Horticultural Society 

 Scottish Meteorological Society... 



PAGE 

 116 

 116 



109 



Artificial Reproduction of Volcanic 

 Rocks 



116 



no 



Correspondence : 



A Circle Problem — Aurora 

 Borealis ... ... 



11S 



112 

 112 

 "3 



114 

 »5 



Recent Inventions 



Technical Educational Notes... 



Sales and Exchanges 



Selected Books 



Notices ... 



Meteorological Returns 



11S 

 119 

 120 

 120 

 120 

 120 



SCIENTIFIC TABLE TALK. 



By W. Mattieu Williams, F.R.A.S., F.C.S. 

 Another series of Mr. Spitta's experiments were made 

 by dissecting, so to speak, the representative planet, 

 cutting a hemispherical shell into first a central segment 

 of 30 degs., then a series of annular segments or zones 

 from 30 degs. to 60 degs., from 60 degs. to about 70 

 degs., and from 70 degs. to 90 degs., and making transits 

 over these. The results of these experiments, briefly 

 stated, were that when the little moon with albedo of 

 o'i3 advanced towards the artificial planet with the cen- 

 tral segment in situ, or removed, no apparent change was 

 due to this removal ; the same occurred when the 

 moon was placed on the limb ; and the same again 

 when the moon was placed on the zone at about 

 the angle of 65 degs. to 70 degs.; in like manner the 

 removal of second zone made no difference; but when 

 the moon was placed en a portion corresponding to 

 about the angle of 60 degs. it appeared exceedingly black 

 while the central portion remained, but recovered most of 

 its brilliancy when the central portion was removed, it 

 then merely appeared " rather more tinted than usual; " 

 when the second as well as first zone removed, the 

 moon recovered its original brilliancy. 



I must not be tempted into proceeding with further 

 details, but must refer any readers who desire to study the 

 subject further to the Monthly Notices of the Royal 

 Astronomical Society of November, 1887, where they 

 will find the original paper. I have no doubt that all 

 who read it carefully will agree with me in regarding it 

 as a very able and important contribution to physical 

 astronomy and optical science, worthy of far more atten- 

 tion than it appears to have hitherto received. 



It will be noted, of course, that in both cases of planet 

 and satellite we are dealing with spheres that reflect the 

 radiations from the sun. If the surface of the orb were 

 perfectly polished all the sides curving away from us 

 would be invisible ; only the central portion would be 

 seen, and this, if sufficiently magnified, would present 

 to us a mirrored image of the sun. 



Thus, a moon as big and as near to us as our existing 

 satellite would display merely such an image diminished 



to a brilliant speck of light. That we are able to see the 

 hemispherical rotundity is due to the kind of reflection, 

 the " scattering " reflection, which is common to all 

 ordinary surfaces, and by virtue of which ordinary 

 objects become visible in their actual form. If all the 

 objects in a given apartment had perfectly polished sur- 

 faces, and a gas flame were the source of light, nothing 

 more than a multitude of gaslights would be visible. 



But the sun is spherical, and is visible by virtue of 

 his own luminosity. How is this apparent luminosity 

 affected by the obliquity of various zones in reference to 

 our line of vision ? 



This question has been the subject of many experi- 

 mental investigations. Omitting those relating to the 

 spectrally dissected rays, I may quote Pickering's results 

 for the general light. Calling the sun's centre 100, and 

 dividing the radius of the visible disc from centre to 

 limb into 100 parts, he found that at 10 parts distant 

 from the centre the luminosity was reduced to 988, at 

 40 to 94, at 50 to 9i - 3, at 60 to 87-0, at 75 to 78-8, 

 at 85 to 69-2, at 95 to S5"4, and at 100, i.e., at the very 

 edge of the limb, to 37 "4. 



From this it appears that the loss of light due to 

 obliquity is far less than in the case of Spitta's ball of 

 plaster of Paris. 



Our mathematicians affirm, on theoretical grounds, 

 that " an incandescent sphere of metal or an illuminated 

 globe of white glass (like the shade of a student-lamp) 

 appears sensibly of equal brightness all over, the fore- 

 shortening of each square inch of surface inclined to the 

 line of sight just compensating for its diminished radia- 

 tion " (" The Sun," by Dr. Young, chap, viii.) ; and they 

 attribute the diminishing luminosity of the different 

 zones of the sun, proceeding towards the limb, to the 

 absorption of his light by his own atmospheric envelope, 

 this absorption increasing, as it evidently must increase, 

 with the obliquity, simply because we are looking 

 through a greater depth of such atmosphere when our 

 line of sight is oblique than when it is perpendicular. 

 This difference corresponds to that of our daily experi- 

 ence in our own atmosphere. The sun appears far less 

 bright on our horizon than overhead, because when on 



