56 Proceedings of the Royal Physical Socvety. 
of bright sunshine, or better, the percentage of possible 
duration, is a fair measure of the atmospheric transparency. 
When this percentage rises, as in the spring of 1890, to as 
much as 33 (one-third of possible duration), the transparency 
of the air may be regarded as unusually high. 
In comparing the records of different localities with a view 
to estimating the atmospheric state of each, a correction must 
be introduced for differences in the depth of atmosphere 
traversed. At- Strathpeffer, for example, the solar rays, 
coming obliquely from their source. at a comparatively low 
altitude, traverse a wider belt of atmosphere than is the case 
farther south. It follows that the same percentage of sunshine 
implies a greater transparency of the atmosphere the farther 
north it is recorded ; that is, the longer and more oblique its 
course through the aérial envelope of our planet. Now at 
Strathpeffer and London the winter percentages of possible 
duration of sunshine are respectively 19:2 and 8, or very nearly 
as 24 to 1. When to this difference is added the further 
difference arising from the greater depth of the atmosphere 
traversed at the more northerly station, it is safe to conclude 
that the transparency of the air is at least three times greater 
at the Spa than in the neighbourhood of London. 
V. On the Fructification of Sphenophyllum trichomatosum, 
Stur, from the Yorkshire Coal Field. By Ropert 
Kipston, F.R.S.E., F.G.S. [Plate I.] 
(Read 15th April 1891.) 
The earliest figure of Sphenophyllum is probably that given 
in 1804 by Schlotheim in his “ Beschreibung merkwiirdiger 
Kraiiter-Abdriicke und Pflanzen Versteinerungen,” pl. i1., fig 
24, and described on p. 57. Schlotheim here supposes his 
fossil to belong to the palms. In his “ Petrefactenkunde” 
(1820-23) he named his specimens Palmacites verticillatus, 
In 1822 Brongniart created the genus Sphenophyllites for 
these fossils,! and in his “ Prodrome”? he names Palamacites 
1 Sur la Classification et la Distribution des végétaux fossiles en général, 
etc., in Mem. du Muséum (hist. nat., vol. viii, p. 34, pl. ii, fig. 8, 
Sphenophyllum emarginatum. 
21828, p. 68, 
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