456 



PHILOSOPHICAI, TRANSACTIONS. 



[anno 1767. 



water. Where the coral banks are not exposed to the common monsoon, they 

 will alter their direction ; and be either round, or extend in the parallel, or be 

 of irregular forms, according to accidental circumstances. 



The interior parts of these islands, being sea, sometimes form harbours 

 capable of receiving vessels of some burthen, and he believes always abound 

 greatly with fish; and such as he has seen, with turtle-grass and other sea-plants, 

 particularly one species, called by the Sooloos gammye, which grows in little 

 globules, and is somewhat pungent as well as acid to the taste. It need not be 

 repeated that the ends of those islands, only, are the places to expect soundings: 

 and they commonly have a shallow spit running out from each point. Abdul 

 Roobin's observation points out another circumstance, which may be useful to 

 navigators; by consideration of the winds to which any islands are most exposed, 

 to form a probable conjecture which side has deepest water; and from a view 

 which side has the shoals, an idea may be formed which winds rage with most 

 violence. 



XXXVIII. An Attempt to determine the Height of the Suns Atmosphere from 

 the Height of the Solar Spots above the Suns Surface. In a Letter to 

 Mr. J. Ellicot, F. R. S.from the Rev. Mr. Samuel Horsley, F. R. S. p. 398. 

 I return you many thanks for your obliging communieation of the observa- 

 tions of the late transit of Venus by Mayer and Rohlius. The phenomena 

 which they relate of the atmosphere of that planet are highly curious. They 

 were perhaps the more interesting to me, as they confirmed some conjectures 

 of my own concerning the great height of the atmosphere of the sun, and of 

 those of the two nearer planets. I once attempted to make a rough comparison 

 between the height of the sun's atmosphere and that of our own, by comparing 

 the height of the solar spots above the surface of the sun with that of our clouds 

 above the surface of the earth, which I did in the following manner. 



The inclination of the sun's equator to the place of the earth's orbit is so 

 small, that in this inquiry I think it may safely be neglected; and I consider the 

 two planes as one. Let t be the centre of the earth, s that of the sun. Join 

 TS, and let def be a great circle of the sun's sphere, formed by the intersection 

 of the plane of the earth's orbit with the sun's surface. Let abc be the circle 

 described by the revolution of a spot. From t draw if and Td touching the 

 circle def in f and d, and cutting 

 ABC, in c and a. Join df; 

 through s draw hsg, parallel to 

 df. Join sc, SA, sd. The spots 

 are hid behind the sun 3 daysB' 

 longer than they are visible. 

 That is, they are hid 15 days, and 

 are seen only 12. The earth's 



