306 Royal Society. 
strong to resist the forces to which they are peculiarly exposed: 
and he finds upon ,calculation that Mr. Uppings’s braces are 
likely to be strong enough to resist any force which ean easily 
be applied to them. He concludes by recommending the cau- 
tious adoption of these improvements, the most decided of which 
appears to be the filling up between the timbers in the hold, as 
both increasing the strength, and materially tending to prevent 
the decay: and even the obliquity of the planks of the decks, he 
thinks, may in some cases be of advantage, especially when it 
becomes neccessary to have the ship hove down on her side. 
March31. A paper by S. Groombridge, Esq. was read on Atmo- 
spherical Refraction, correcting Observations on the circumpolar 
Stars by the barometric Pressure and thermometrical Temperature. 
This may be considered as a supplement to the author’s for- 
mer paper on atmospherical Refraction ; wherein he had pro- 
posed a certain formula for the computation of the quantity, 
from observations made to less than S0° of zenith’ distance. 
Having since applied this formula from 80° to 89° 42’, he has 
found that the refraction so computed exceeds that observed 
from 80° to 88°; and below 88° gives the quantity less than 
from observation. He now proposes another formula, which will 
agree with the observed quantity to 87°, and, corrected in a cer- 
tain ratio, thence to the horizon. f 
The computation of the refraction, with the corrections for the 
barometer and thermometer, from the tables prescribed by the 
author, will be very direct and simple, and obtained without 
the tedious process of logarithms. In the result they will agree 
nearly with the French tables, to within 2° of the horizon ; when 
both the observation and the refraction become uncertain. 
Part of a paper by Dr, Olay was read, on that part of per- 
spective which relates to circles as distinct from conic sections. 
he author acknowledged his obligations to Mr. Kerrick, the 
hbrarian of Cambridge University ; stated candidly what was ori- 
ginal, and what was only digested from others; and laid down 
Six propositions, only a part of which were of a nature to be read. 
The Society then adjourned, in consequence of the holidays, till 
April 21. The Right Hon. President in the chair. The first 
part of a letter from Dr. Brewster to the President was read, 
detailing some further experiments and curious discoveries on 
light and colours, particularly some optical effects of mother of 
pearl. The prismatic colours exhibited by mother of pearl have 
always been considered as furnishing a good démonstration of the 
Newtonian theory of colours, by its diversified lamine; and Dr. 
B’s experiments do not contradict this explanation. He found 
that mother of pearl polarizes light as well as agate and other 
bodies, in addition to which it has the singular property of com- 
municating 
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