414 
before, and a given stress in any one of the nine will produce 
stresses in each of the other eight which are determinate in 
magnitude when the configuration of the linkage is given, 
though they alter during the motion of the linkage. 
March 13, 1876. 
THE PRESIDENT (PROFESSOR CLERK MAXWELL) in the Chair. 
The following communication was made to the Society : 
On a set of Lunar Distances. By Mr Prarson. 
In this paper I propose to discuss the anomalies exhibited by 
a set of Lunar distances which I took under rather peculiar cir- 
cumstances last autumn. The sky was perfectly clear, with the 
exception of a light cloud touching the sun, at the commence- 
ment: and the horizon quite open. The position of observa- 
tion was, by the ordnance survey, as near as possible Lat. 
52°.7°.13" N., Long. 56*.(14°) E. The instrument with which 
they were taken is the same prism-circle, 6 inches in diameter, 
by Pistor and Martins, of Berlin, which was shewn on a pre- 
vious occasion at a meeting of the Society, and with which I 
took the observations given pp. 351—354, and pp. 357—359 of 
this vol. of the Proceedings. 
It will be well first to consider the condition of the instru- 
ment itself, as it may be suggested that the source of the 
anomalies is to be found here. Being graduated all round, the 
mean of the opposite readings has in all cases been taken. 
Between these there is, in the first case, a discrepancy between 
the opposite readings of two divisions of the vernier, or 40” 
about; for (2) we have a discrepancy of 35‘; (3), (4), (5), and - 
(7) give the same readings on both sides of the arc; (6) gives a 
discrepancy of 20°. At 101° and at 102° there is no discre- 
pancy between the opposite readings: so that it is clear that 
