ATMOSPHERIC PRESSURE IN PAST AGES.

King Country Chronicle, Volume VII, Issue 610, 11 October 1913, Page 7

ATMOSPHERIC PRESSURE IN PAST AGES.

WHY AEROPLANES MUST GLIDE —NOT FLY.

The modern development of aviation suggests the question whether the atmosphere of the earth was denser during the carboniferous opoch than at the present o.;y.

"Among the animals which people the earth," says a Frmcli authority who has just spoken on the subject, "we find representatives of all ci?es, from the microscopically sir.all animalcule to the great elephant. But within the limits of any cm species the scale upon which the individuals are built does not tii :Ter very materially : we do n r .>t. for example, find specimens, s.-.y, tea times larger than their adult congeners. 'Tt is easy to show goo;! r.-.ccha-nical and physical reason why this should be so. Consider two birds geometrically similar, such as two swallows for example, and imagine the second to have t r n times the normal size. This bird wi'l then have all its linear dimensions ten times as great as the normal bird, while the surfaces will be one hundred times as great. In this way all the cross sections, the thickness of the muscles, especially of tlis winy muscles,' being increased ons hundred fold, we may safely take it that the strength of the bird will also be increased in the same proportion. '"But on the other ,hand, it must be remembered that the volume, and hence the weight of the body, will be increased one thousand fold. Thus; this' large swallow, built on ten times the 'normal scale, would find itself quite unable* to rise into the air or to sustain itself there, for it is relatively ten times less strong than its congeners : with muscles one hundred times as strong- as theirs it 'is called upon to perform one thousand times the work. "As a matter of fact, an examination of the flight of different kinds of birds having approximately the same form shows that flight becomes more and more difficult as the weight increases ; large birds substitute as iar as possible sailing flight (which is the characteristic motion of our aeroplanes) for flapping of the wings. Thus the size of animals capable of flight has an upper limit, and this seems to be reached in the present state of nature, by the large birds, so far as sailing flight is''concerned, and by the large insects, so far as flight by wing vibration is concerned. " And yet in past ages niv.ch greater animals have flown. Cne reptile of the group Pterodactyl had a span 'of over thirty feet, v. hi:h exceeds that of a racing Blerint ; this creature lived during tin cretaceous period and flew as fcr as 90 miles inland. Certain dragon flies of the carboniferous era measured over three feet from tip o tip of their outstretched win-s. Under present conditions 'it would be quite impossible for these crea tures to fly. '■' The most natural supposition is that in the times when tl:c:c creatures flew through the air, the atmosphere had- a greater density than it has at present. In the estimation of Mr. Harle, the existence of these great flying an'imals during cretaceous and carboniferous times indicates that atmospheric pressure at that time was greater than at present. "J-"' Popular Science Sittings."