Sept. 22, 1882.J 



KNOWLEDGE ♦ 



279 



turned edgewise to the eye ; P the fixed luminous point at 

 a given distance PF from the plane ; ABF the sphere so 

 situated that its centre C is seen projected on the perpen- 

 dicular PF ; PBE, PAD planes touching the sphere, their 

 line of intersection being a straight line through P parallel 

 to the plane DE, and seen as a point at P. Then, obviously, 

 DE is the breadth of the shadow of the sphere ABF, and 

 as obviously DE is constant. For 



FE'" : CB^ :: PF^ : PB- :: PF^ : PC-- CB^ 

 and CB, PF, and PC are given constants. Thus, the 

 breadth or lesser axis of the elliptical shadow of the sphere 

 is constant wherever the sphere may be. 



We may regard the spherical clouds in Fig. 1 as equal 

 spheres, all touching the horizontal plane AZ ; for the 

 luminous point we may set an eye at O ; and we have 

 learned that the apparent projection of any of the clouds 

 on the plane AZ is an ellipse whose shorter axis is 

 ■constant, while its longer axis, of course, becomes greater 

 and greater the farther the spherical cloud is from Z. 



Now, if a blue plane AZ were occupied by a number of 

 •equidistant white circles, an eye at O would recognise the 

 same proportion of whiteness in all directions, for the 

 white circles and the blue spaces between would all be 

 foreshortened and reduced in equal degree towards 6, 7, 8, 

 •or A. But since the spherical clouds are not projected 

 into circles all of the same size on the blue blackground, 

 but into ellipses all of equal breadth, and longer and longer 

 the further they are from the centre, there is a propor- 

 tionate increase of white sky with increase of distance from 

 the point overhead. 



Thus, if we suppose a horizontal layer, as in Fig. 1, 

 really covered with clouds arranged as in Fig. 3, the pro- 

 jection of these on the horizontal plane AZ, would be as 

 shown in Fig. 4. But, as seen by the eye, the proportions 

 of this projection would all be so reduced radially outwards 

 that all the ellipses would become circular (for in whatever 

 way we look at a sphere it always looks perfectly round), 

 and we should have the arrangement shown in Fig. 5, 

 where the relative whiteness of the general surface is the 

 same for each part as for corresponding parts of Fig. 4. 



Fig. 5 shows the law according to which the sky gets 

 more closely cloud-co\ered with distance from the point 

 overhead, though the individual stars get smaller. Tt 

 shows the sky to a distance of about T) deg. from the 

 zenith, tliat is, half way down to the horizon. 

 {To be continued.) 



COMPOUND NEST. 



By Daniel C. Be.\kd (in the Scientific American). 



ALMOST every one whose business or occupation has 

 introduced him to an intimate acquaintance with the 

 salt marshes that line our eastern coasts is familiar with 

 the odd, chattering notes of the marsh wren. This little 

 bird finds its board and lodging among the reeds and rank 

 grasses of the damp, salt meadows. Morning and evening 

 its song, if such vocal eflbrts can be so called, may be 

 heard, but especially does it delight to sing at night. Often 

 after a long sail, when belated and overtaken by night, the 

 writer has welcomed the harsh, but not unpleasant, notes 

 of the long-billed marsh wren {Cislothcrus palustris) as a 

 signal from shore and home. 



Not long since a boating party caught in a dense fog 

 only discovered their dangerous proximity to the shore 

 from the warning notes of one of these little coastguards. 



Componml Nest of tlic C. P.ifvstr 



Other birds find refuge and sustenance among the salty 

 sedges inhabited by the marsh wrens. Among them may 

 be seen the brilliantly-decorated Agefaitis p/i(r7iicfus, com- 

 monly known as the red wing or swamp blackbird. The 

 lustrous black plumage of the male bird shines in the sun, 

 giving out greenish metallic retlections. Its shoulders and 

 lesser wing coverts are ornamented with crLuison epaulets. 



