144 PHILOSOPHICAL TRANSACTIONS. [aNNO J777. 



the first-mentioned brothers, vvlio is now living, is master of a trading vessel 

 belonging to Mary-port. Mr. H. met him in December 1776, at Dublin, and 

 took the opportunity of conversing with him. He wished to try his capacity to 

 distinguish the colours in a prism, but not having one by him, he asked him 

 whether he had ever seen a rainbow ? He replied, he had often, and could dis- 

 tinguish the different colours ; meaning only, that it was composed of different 

 colours, for he could not tell wliat they were. Mr. H. then showed him a piece 

 of ribbon: he immediately, without any difficulty, pronounced it a striped and 

 not a plain ribbon. He then attempted to name the different stripes: the 

 several stripes of white he uniformly, and without hesitation, called white: the 

 4 black stripes he was deceived in, for 3 of them he thought brown, though 

 they were exactly of the same shade with the other, which he properly called 

 black. He spoke, liowever, with diffidence as to all those stripes; and it must 

 be owned, the black was not very distinct: the light green he called yellow; but 

 he was not very positive: he said, " I think this is what you call yellow." The 

 middle stripe, which had a slight tinge of red, he called a sort of blue. But 

 he was most of all deceived by the orange colour; of this he spoke very con- 

 fidently, saying, " This is the colour of grass ; this is green." Mr. H. also 

 showed him a great variety of ribbons, the colour of which he sometimes named 

 rightly, and sometimes as ditterently as possible from the true colours. 



The experiment of the striped ribbon was made in the day-time, and in a 



good light. ,,.■',■. , ,;;.<• 



Xf^. A new Theory of the Rotatory Motion of Bodies affected by Forces 

 Disturbing sitch Motion. By Mr. John Landen, F. R. S. p. 266. 



I consider this paper as not unworthy the notice of this society, through a 

 persuasion that the theory contained will conduce to the improvement of science, 

 by enabling the reader to form a true idea, and accordingly to make a computa- 

 tion of the motion (or change) of the axis about which a body having a rotatory 

 motion will turn, or have a tendency to turn, on being aflected by a force 

 disturbing its rotation; particularly of the motion of the earth's axis arising 

 from the attraction of the sun and moon on the protuberant matter of the earth 

 above its greatest inscribed sphere: which compound motion has not been rightly 

 explained by any one of the eminent mathematicians whose writings have come 

 to my hands. , . 



1. Let the sphere adbe, fig. 12, pi. 1, whose radius is r, revolve uniformly 

 about the diameter acb as an axis, with the angular velocity c, measured at d 

 or E, the motion being according to the order of the letters dgeh, in the 

 section at right angles to acb, fig. 13; and while it is so revolving, let the pole 

 A be impelled by some instantaneous percussive force to turn about the diameter 



