of Edinburgh, Session 1872-73. 
55 
4. Note on the Zodiacal Light. By George Forbes, Esq. 
A peculiarity was observed about the vernal equinox in 1871 
in the shape of the zodiacal light, which deserves to be recorded. 
The appearance resembled a thin cone (such as is usually seen), 
extending to a great height, and rising out of a broad low cone 
situated at its bj.se. This was not an effect of sunlight, for it was 
visible hours after sunset. It was not peculiar to any time or 
place, for it was seen constantly in all parts of the south of Europe, 
viz., in the Bay of Biscay, all along the Mediterranean, in Malta, 
and in Sicily. It seems not unlikely that there are periodic 
changes in the appearance of the zodiacal light. Hence it is well 
to mention any such peculiarity. I have also to confirm what has 
so often been stated by other observers, that the direction of the 
axis of the cone is not always in the direction of the ecliptic, but 
changes its direction from night to night. 
Monday, 11th February 1873. 
Sir ROBERT CHRISTISON, Bart., President, 
in the Chair. 
The following Communications were read : — 
1. Note on Angstrom’s Method for the Conductivity 
If we assume the excess of temperature above that of the air, v, 
to be the same throughout a transverse section of the bar, the 
equation for the flux of heat is — 
where cp is the water equivalent of unit volume of the bar, h its 
thermal conductivity, a its side, and hv the quantity of heat lost by 
radiation and convection from unit surface of the bar per unit of 
time, when the excess of temperature is v. 
of Bars. By Professor Tait. 
VOL. VIII. 
