Planetary science: Tunguska at 100
The most dramatic cosmic impact in recent history has gathered up almost as many weird explanations as it knocked down trees, writes Duncan Steel.
The fragmentation of small asteroids in the atmosphere
The fragmentation of a small asteroid in the atmosphere greatly increases its cross-sections for aerodynamic braking and energy dissipation. The differential pressure across a meteoroid disperses its fragments at a velocity that increases with atmospheric density and impact velocity and decreases with meteoroid density. At a typical impact velocity of 22 km/s, the atmosphere absorbs more than half the kinetic energy of stony meteoroids with diameters, D M < 220 M AND IRON METEOROIDS WITH D M < 80 m. The corresponding diameter for comets with impact velocity 50 km/s is D M < 1600 m. Most of this energy dissipation occurs in a fraction of a scale height, which causes large meteoroids to appear to »explode« or »flare« at the end of their visible paths
THE GEOLOGICAL SURVEY IN YORKSHIRE
( Presidential Address delivered at Sheffield , 14 th December, 1957).

I. General History For some years after its institution in 1835 the Geological Survey was preoccupied with the south-western parts of Britain, and it was almost mid-century before the work of “making the geological map” was extended northwards to the borders of Yorkshire. Already several one-inch maps had been completed and were issued, hand-coloured, on what later became known as the “Old-Series” grid, and in 1846 De la Beche published his first volume of Memoirs of the Geological Survey. We find the name of Warington W. Smyth on a detailed section, dated 1845, of the first Woodhead Tunnel which lies partly in Yorkshire. And at about the same time William Smith’s nephew, John Phillips, was spending some eight years of his life on the staff of the Geological Survey, bringing to publication in 1852 the first official map to cover a part of Yorkshire—the original edition of the Old-Series sheet 81 NE., mainly covering the High Peak of Derbyshire but including, on its east side, small parts of the Yorkshire Coalfield near Sheffield.

Once started, the survey of Yorkshire continued at such a pace that its completion was announced in 1882. A small band of field geologists of outstanding ability was responsible for this feat: W. W. Smyth, A. H. Green, C. le Neve Foster, Clifton Ward and J. R. Dakyns who, with John Phillips, brought the survey of the Sheffield area, Sheet 82 NW., to its publication in 1867; W …