86 



NA TURE 



[May 24, 1900 



two American Black Snakes {Zamenis constrictor), ten Penn- 

 sylvanian Mud Terrapins {Ctnosternnni pennsylvanicum), four 

 Adorned Terrapins {Chrysemys orna'-a), thirteen Elegant 

 Terrapins [Chrysemys scripta elegans), six Lesueur's Terrapins 

 {Malacoclemmys lesiteuri), six Red '^t^\.%{Sperlepes ruber) from 

 North America, a Garnett's G3.\a.go [Galago garnetti) from East 

 Africa, a Serval (Felts servxl) from Africa, a Common Teguexin 

 ( Titpinambis teguexin), three Annulated Terrapins {Nicoria 

 annulata) from South America, four Blue Wall Lizards {Laceria 

 mwalis, var. co'rule%) from Faraglione, five Schlagintweit's 

 Frogs {Rana cyauophlyctis) from Southern Asia, deposited ; a 

 Barbiry Wild Shesp (Ovis iragelaphiis, i), born in the 

 Gardens. 



OUR ASTRONOMICAL COLUMN. 



The Dark Fringes observed during Total Solar 

 Eclipses.— We have received a communication from Senor V. 

 Ventosa, astronomer at the Madrid Observatory, concerning the 

 appearance and probable cause of the dark fringes — or "shadow- 

 bands " as they are generally called— which are always observed 

 some few seconds before and after totality during the progress 

 of a total eclipse of the sun. The chief points of his com- 

 munication are here summarised. 



These alternating dark and bright fringes are parallel to each 

 other, all moving in the same direction, but the velocity 

 varies greatly from time to time. Several reasons have been 

 advanced to account for their appearance, chief of which are 

 those regarding them as (i) diffraction fringes bordering the 

 actual shadow of the moon on the earth's surface ; (2) shadow 

 phenomena produced in the body of our own atmosphere, and 

 affected by the direction of the wind. The examination of the 

 observed facts appears to support to some extent those holding 

 the latter view, as while the bands may be well seen in one 

 place, they may be invisible in a neighbouring locality ; their 

 form, generally rectilinear or slightly undulating, is also variable, 

 while their breadth has been variously estimated from i cm. to 

 50 cm., although this will, of course, partly depend on the in- 

 clination of the surface on which they are observed. Sometimes 

 they move with about the speed of a man walking, at others 

 with the speed of an express train, the velocity always being 

 less, however, than that of the shadow itself. (During the 

 coming eclipse the shadow will move through 800 kilom. in 

 12 minutes.) 



Senor Ventosa has been occupied for some time in study- 

 ing the currents in the higher regions of our atmosphere by 

 observing the undulations round the sun and stars with a 

 telescope, and thinks that these upper atmospheric currents 

 may possibly have some bearing on the question of the eclipse 

 shadow bands ; the movement of these higher portions show- 

 ing through the quieter lower strata and being rendered visible 

 on account of different refractive powers. He thinks it would 

 be useful to determine the velocity of these currents by 

 anemometers at various altitudes, and also to observe the 

 undulations round the limb of the sun at the time of eclipse, 

 comparing them with the shadow bands in direction and 

 velocity of movement. To ascertain if any experimental 

 illustration of this hypothesis could be presented, he states 

 that bands may be produced by passing diffuse light reflected 

 from a sheet of corrugated glass through a circular aperture 

 representing the 5un, over which an opaque disc, represent- 

 ing the moon, is made to slide. When the segment left un- 

 covered is about 5 mm. in width, alternate bright and dark bands 

 can be observed on a white screen held near, if the length of the 

 segmental opening is approximately parallel to the undulations 

 of the glass, but if at right angles they entirely disappear. He 

 trusts, however, that his putting forward this hypothesis for 

 establishing a connection between eclipse shadow bands and 

 atmospheric undulations will show the advisability of recording 

 the direction and velocity of the wind during eclipses, so that 

 more definite data may be available for discussion. 



Photometry of Corona, April 16, 1893.— In a com- 

 munication recently made to the Royal Society, Prof. H. H. 

 Turner, F.R.S., gives the details of procedure and results 

 obtained in photometric observations of the corona during the 

 total eclipse of the sun in April 1893. The visual brightness of 



NO. 1595. VOL. 62] 



the corona was determined by Prof. T. E. Thorpe at the eclipses 

 of i$86 August 29, and 1893 April 16, by a method arranged by 

 Sir W. Abney {Phil. Tratts. A, 1889, p. 363, and 1896, p. 

 433), but soon after the first of these. Sir W. Abney devised a 

 method of comparing the coronal light with that of a fixed 

 standard by photographic means. This method was first put 

 into practice at the eclipse of 1889, and has been repeated 

 systematically since. Part of the photographic plate, before 

 being taken for eclipse use, is exposed to a graduated series of 

 exposures from a standard source of light in the laboratory, 

 and then without development is afterwards used to receive 

 the impression of the corona, the part carrying the pre- 

 vious standard exposures being protected from further light 

 action. On subsequent development there results a picture of 

 the corona, and ,a series of squares of graduated densities on 

 the same plate, so that the brightness of any part of the coronal 

 structure may be directly compared with the brightness of the 

 standard light of the laboratory. 



The 1889 photographs have not yet been measured, but Prof. 

 Turner has reduced several of the plates taken in 1893 by 

 Sergeant Kearney at Fundium, Africa. These were obtained 

 with the *' double tube " apparatus, giving pictures of two sizes, 

 the moon's disc being 06 inch and 15 inches in diameter. 

 Examples of each scale image were examined, one of the large 

 scale photographs, taken with an exposure of 50 seconds, being 

 specially carefully measured along four radii extending due 

 N., S., E., W. , from the limb respectively, and the resulting 

 table of comparison measures is included in the present paper. 

 This table shows : — 



(i) That the accuracy of the method is such that the intensity 

 of the light is determinable within a very small error. 



(2) The intensity of the coronal light falls off in nearly the 

 same manner in all four directions (1893 was near a sun-spot 

 maxiviiun, with corona of symmetrical form). There is a 

 marked difference, however, between the intensities along the 

 north and .south radii. 



(3) The falling off in intensity is at first exceedingly rapid, 

 becoming very gradual at distances more than 45 minutes from 

 the limb. 



(4) The absolute brightness of the corona in terms of the 

 " moon " by using a conversion factor. 



Prof. Turner then compares the brightness thus determined 

 photographically with that obtained visually by Abney and 

 Thorpe, and presents two curves showing the combined obser- 

 vations, which show a marked agreement between the results 

 arrived at in such different ways. No measures of brightness, 

 however, were made visually within o'6 of a radius from the 

 limb, and it would be useful if this were done at the coming 

 eclipse. 



Maximum Duration of Totality for Solar Eclipse.— 

 Mr. C. T. Whitmell sends us the following corrections to the 

 data given in the abstract of his paper last week (p. 64) : — 

 Earth's radius to be taken as 3963*296 miles. 

 Moon's ,, ,, ,, „ ,, 1080 '000 ,, 

 The eclipse for which the totality will be a maximum will take 

 place at noon about the beginning, not the middle, of July. 



SOME MODERN EXPLOSIVES.^ 



"M" EARLY thirty years ago, in the Royal Institution, I had 

 -'■ the honour of describing the great advances which had 

 then recently been made both in our knowledge of the phenomena 

 which attend the decomposition of gunpowder, and in its prac- 

 tical application to the purposes of artillery. 



I described the uncertainty which up to that date had existed 

 as to the tension developed by its explosion, the estimates 

 varying enormously from the 101,000 atmospheres (about 662 

 tons on the square inch) of Count Rumford to the 1000 atmo- 

 spheres (6"6 tons per square inch) of Robins, or, taking more 

 modern estimates, from the 24,000 atmospheres (158 tons per 

 square inch) of Piobert and Cavalli to the 4300 atmospheres 

 (about 29 tons per square inch) of Bunsen and Schischkoff. 



These uncertainties were, I think I may say, set to rest by 

 certain experiments carried out both in guns and close vessels 

 at Elswick, by the labours of the Explosive Committee appointed 



1 A Discourse delivered at the Royal Institution on Friday, March 23, by 

 Sir Andrew Noble, K.C.B., F.R.S. 



