390 Proceedings of the Royal Society of Edinburgh. [Sess. 
whole on a vertical iron pipe L (fig. 3), of about 10 cm. bore, firmly 
fixed to the roof and rising at least 1J metres above it. The upper end 
of this iron pipe is closed except for the tubular opening M, which 
transmits the hollow axis of the vane and which forms a bearing for the 
latter. Inside the iron pipe a smaller concentric pipe P is fixed for a 
considerable distance, the two being connected at the bottom of the latter 
so that the space between can be filled with water. To the vane axis a 
concentric double-walled tubular float Q is attached which dips in the 
water so that the weight of the vane and all that it carries may as far as 
possible be supported thereby. 
