282 
As soon as the observer has entered this + 20, in his field- 
book, the assistant comes on and stands where the observer did, 
whilst the observer goes on and takes another observation. 
In order to determine the difference of Level between any two 
stations he has only to take the algebraical sum of the No. 
of feet recorded in his field-book. 
Instead of the glass tube, a second gauge might be em-. 
ployed, and the difference of readings of the gauges entered as 
the differences of level at each step. In this case the gauges 
need not be constructed to read below zero. 
This method of Levelling would be extremely rapid and 
sufficiently accurate for most practical purposes. The instru- 
ment would be inexpensive, and could be used by an un- 
educated observer. It could also be employed at night, which 
would render it useful in military work before an enemy. 
By using mercury instead of water, and a very fine flexible 
tube of copper wire, the readings might be rendered extremely 
accurate, and the whole weight of the instrument reduced to 
a very few lbs. 
Professor LIVEING communicated a note on Mr Paley’s 
paper, “On effects of light on Portland stone,” read at a former 
meeting. During the summer he had visited Portland, and 
had found that all the stone there became black on all old 
exposed surfaces. He had sent specimens of the black part to 
Mr Berkeley, from the Portland quarries and from King’s 
College Chapel; who said that they were the early stage of 
some lichen. Professor Liveing had found this blackness on 
St George’s Church on the top of Portland, but only on surfaces 
exposed to rain drip—the tombstones also were discoloured. 
Mr Berkeley had informed him that the lichens in the two 
cases named above were not the same species. The stones at 
Portland also occasionally had a red lichen growing on them. 
New Fellow elected: H. W. Witson, Hon. M.A. 
