i879-1 
Proceedings of Societies, 
383 
short distance from each tower, by a succession of increasing 
angles of vertical divergence, so that the illumination of the sea 
became gradually diminished as the distance from land was 
lessened. In each light there was a rearward arc to spare, and 
this was turned to valuable account, from 67 to 71 per cent of 
this light being cohered and acfted upon by optical agents, which 
were particularly described, and thereby distributed uniformly 
over the front azimuthal arc, so as to intensify not only the illu- 
mination of the horizon and the distant sea, but also that of the 
nearer sea. It was mentioned that the two Lizard Lights, which 
were both fixed, and were first exhibited in March, 1878, had 
optical arrangements similar in every respecft to those adopted at 
the South Foreland Lighthouses, with a slight variation in the 
refracting portions, arising from the circumstance that existing 
apparatus had to be turned to account in the construction of 
each apparatus. A table was appended, showing the condensing 
powers in the direction of the horizon of the lights described in 
the paper, distinguishing those optical portions which parallelised 
the incident light from those which gave to it special vertical 
divergence. According to this table, upon the assumption that 
the diameter of the electric arc was 12 millimetres, the con- 
densing powers in the sea-horizon direction were as follow : — 
Souter Point — Revolving ... 
South Foreland — High fixed 
South Foreland — Low fixed 
Two Lizard Lights — Fixed... 
236-38 
5 0 * 1 7 
43-40 
58-44 
and data were added for adapting this table to particular cases. 
In a second table was given the respective condensing powers 
over the near sea, at different distances from the lighthouse 
towers. 
Meteorological Society, February 19. — Mr. C. Greaves, 
President, in the chair. 
Among the papers read were the “ Diurnal Variations of Baro- 
metric Pressure in the British Isles,” by Frederick Chambers. 
The object of this paper is to show that differences of types of 
the diurnal variations of pressure at inland or sea-coast stations 
are due to the superposition, on a common type of diurnal varia- 
tion at all the stations, of a distinct diurnal variation of barometric 
pressure, such as is required to satisfy the convection-current 
theory which explains the well-known diurnal land and sea 
breezes. To show this, all that is necessary is to take the differ- 
ences of the corresponding hourly inequalities of the barometric 
pressure at pairs of inland and coast stations, and to exhibit 
these differences in the form of curves, which are then found to 
closely resemble the curves of diurnal variation of air tem- 
perature. 
2 C 2 
