Co7'respondence 



373 



Migration.— Mr. Thomas Southwell, F.Z.S., writes: "There can 

 be no doubt that the Grey Crow is in Britain a decided migrant, 

 rarely spending the summer months with us : I have witnessed their 

 arrival repeatedly. The Rook, although most of our home-bred 

 birds are to a great degree sedentary, receives large additions to 

 its numbers in autumn, which depart in the spring, and the same 

 may be said of the Jackdaw and Jay. Migration may be divided 

 into migration proper, i.e., when birds only spend a part of the 

 year with us, and partial migration. In most cases, when certain 

 species are with us the whole year, the home-bred individuals are 

 partial migrants and move on, their places having been taken by 

 birds of the same species from more northerly localities. In this 

 sense there is hardly a single British bird (with the possible excep- 

 tions of the Sparrow and Green Woodpecker) which is not more 

 or less migratory. 



"Amongst the marine mammals and fishes, especially the seals 

 and cetacea, migration is very pronounced. The subject is a most 

 interesting one, and beset with great difficulties." 



Juvenis. — The Air contains oxygen, nitrogen, carbon dioxide, and 

 aqueous vapour mixed together, but not otherwise combined. 



All living things need oxygen, and can, under suitable conditions, 

 receive it from the air and make use of it. 



All green vegetables can take in carbon dioxide, decompose it, and 

 retain, as needed, either the oxygen or the carbon. It is by fixing the 

 carbon that all woody structure is produced, and many other vege- 

 table products. Animals cannot decompose carbon dioxide, nor can 

 fungi. Both fungi and animals are consequently dependant upon other 

 substances for their carbon. 



M.G. — The moon is constantly receding from the earth, and the 

 distance consequently becomes greater every year. 



H. K. R. — It is undoubted that the sun does not directly heat the 

 air. The following quotation will answer your queries better than 

 anything that we can write. " The great aerial ocean which sur- 

 rounds the earth has the wonderful property of allowing the heat- 

 rays from the sun to pass through it without being warmed by them. 

 When the earth is heated, however, the air gets warm by contact with 

 it, and the vapour and carbonic acid in the air are also impervious to 

 the radiant heat given out by the earth, and are therefore warmed by 

 radiation. But the air thus warmed is in continual motion, owing to 

 change of density. It is lifted up and pushed aside by cooler and 

 heavier air; and thus heat can never accumulate in the atmosphere, or 

 warm it beyond a very moderate degree, so that the long-continued 

 sun-heat of the tropics is in great part carried away to give warmth 



3° 



