78 PHILOSOPHICAL TRANSACTIONS. [anNO 1684. 



Ebn Shatir Damascenus, anno Dom. 1363, says, that he corrected the obli- 

 quity, making it 23° 31', not neglecting the sun's horizontal parallax, which he 

 says he found of the extraordinary quantity of 2° 59'. 



Prince Ulnghbeg, anno Dom. 1437, with other astronomers, by using great 

 care and the largest instruments, found it 23° 30' 17" or 23° 30' 27". 



The learned Jew rabbi Moyses ben Maimon, anno Dom. 1174, found it 

 23° 30'. 



I have not consulted half the oriental astronomers, whose writings are pre- 

 served in the libraries of the university of Oxford. But from the above obser- 

 vations, and some others which I reserve to myself, I conclude that the obli- 

 quity of the ecliptic has been always the same from the beginning of the world.* 

 For the later ages, by the assistance of better instruments, have properly cor- 

 rected the error and excess of the ancient astronomy. 



jiccount of a Salt Spring in the Bed of the River Wear, in the County of Dur- 

 ham ; in a Letter to the Editor from Mr. Hugh Todd, Felloiu of University 

 College, Oxon, and Chaplain to the Bishop of Carlisle. N° 163, p. 726. 



When I was at Durham, being informed of an extraordinary salt spring, and 

 another mineral spring, about a mile or a mile and half out of town ; I went to 

 see them the first leisure I had. The salt spring lies on the north-east side of 

 the town, at a place called Salt-water Haugh, near Butterby. It rises in the 

 middle of the River Wear, and is only to be seen and tasted in the summer 

 time, when the river water is discharged all on one side of the channel ; for 

 in winter, when the river is high, it loses its salts in the fresh streams, so that 

 they are not perceivable. The water does not rise in one or two places only, 

 but seems to bubble. up equally in all parts of the channel, for the space of 40 

 yards in length, and about 10 in breadth; for wherever in that space the stones 

 and sand are removed, the water presently springs up. But what I admired 

 most is, that the saltest of all the springs issues out of the middle of a rock. 

 As to the degree of its saltness, it was as high as any brine can be, and though 

 but little in quantity in comparison of the fresh river, yet of that force as to 

 give a brackishness to the streams 100 yards below. Those that have boiled 

 this brine say, that it affords a great quantity of bay salt, not so palatable, yet 



* It is now well known to astronomers that this opinion of Mr. Bernard has no foundation, as the 

 series of observations down to the present time have shown a continual decrease in the obliquity of the 

 ecliptic, at the rate of about 35" in 100 years, by which it has now descended below 23° 28'; and 

 that by calculations on the universal theory of gravitation, it appears that it will continue to decrease 

 only to a certain limit; after which it will increase again to another higher limit, then return, and so 

 continue reciprocating between those two limits continuallj. 



