VOL. L.] PHILOSOPHICAL TRANSACTIONS. 137 



the ship ; which, he observes, is considerably less than he had seen raised by 

 way of experiment. Besides, it may be observed, that the pressure is at least 

 doubled by the friction. 



XXXV. On an Evenings or rather Nocturnal, Solar Iris. By Mr. George 

 Edwards, Librarian of the Coll. of Phys. p. 293. 

 On Sunday evening, June 5, 1757? walking in the fields near Islington, 

 about half a mile north of the upper reservoir or basin of the New River, Mr. 

 E. observed the sun to sink beneath the visible horizon to the north-west, it 

 being very clear in that quarter, except some thin clouds a little above the hori- 

 zon, which were painted of fine red and golden colours, as is usual when the sun 

 sets in a calm clear evening. But about 20 minutes after sun-set, it being then 

 darkish, he saw an iris in the dusky air, at a height greater than is seen at any 

 time in the rainbow. It was in the contrary quarter of the heavens to the setting 

 sun, and fell on the smoke, mists, and evening vapours arising from the city of 

 London and its neighbourhood. The arch seemed to be a full half circle, though 

 its lower parts fell some degrees short of the horizon. It was very distinctly seen 

 for about 1 5 minutes ; its colours the same as in the rainbow, but fainter. The 

 lower ends of the bow arose gradually higher from the earth, as the sun declined 

 beneath the horizon, till the whole arch disappeared. The centre of the arch 

 was aliove the horizon at its first appearance. He could not believe that this 

 arch proceeded from the sun-beams falling on rain : for there had been none 

 that afternoon ; nor was there any sort of signs of rain or rainy clouds to be 

 seen : the wind being northerly, and the air cool, but somewhat hazy in the 

 quarter where the bow appeared. It was not near so bright as the rainbow ap- 

 pears to be in the day time ; and he believes that it would not have been visible 

 at all in the presence of the sun. He imagines it was formed on the gross par- 

 ticles of the evening vapours, mixed with those of the smoke arising from the 

 town. This could not be a lunar arch, the moon being then many degrees 

 below the horizon, and the arch in a place, where it could not be affected by the 

 moon's rays. 



XXXVl. The Effects of the Opuntia, or Prickly Pear, and of the Indigo Plant, 

 in Colouring the Juices of Living Animals. Communicated by H. Baker, 

 F. R. S. p. 296. 



Mr. Baker received a letter from Doctor Alexander Garden, of Charles- 

 town in South Carolina, part of which he laid before the e. s., containing this 



account. 



The Doctor tried the effects of the prickly pear in colouring the urine. He 

 gathered some of the fruit, and gave 4 of the pears to a child of 3 years of age, 



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