448 PHILOSOPHICAL TRANSACTIONS. [^ANNO 16Q\ . 



will come into the same number, having observed no neck in any of them, I 

 shall only add, that these lumbrici hydropici I have always found hanging to 

 the membranous parts, rather than included in the body of any of the viscera, 

 as to the omentum, peritonaeum, or the outward membranes that cover the 

 diaphragm, stomach, liver, colon, or other intestines. And we may be less 

 surprised at the odd structure in this worm, since what I have observed of the 

 lumbricus latus (Philosophical Trans. N° 146,* and of the teres, N° 147^) 

 is as wonderful, though in a different manner. 



On the Fisible Conjunctions of the inferior Planets with the Sun. By Mr. Halley, 

 N° 193, p. 511. Translated from the Latin. 



That Mercury and Venus enter the disk of the sun, and there appear like 

 black spots, is evident, both from the principles of sound astronomy, and un- 

 doubted observations ; but by what laws, or conditions, or in what period of 

 years these phaenomena offer themselves to our view, has not been determined 

 by any of our ffiodern astronomers; on which account I thought it would not 

 be unacceptable to apply seriously to this disquisition, and clear up a subject so 

 perplexed, and so little understood. 



It is self-evident that these phases of these planets always happen in their 

 conjunctions with the sun, when retrograde, viz. when the sun is so near their 

 nodes, that the latitude of the planet, in its conjunction with the sun, does 

 not exceed the semidiameter of the latter. Now, that the limits and conditions 

 of these conjunctions may be the more easily investigated, and since the ele- 

 ments of the calculation are entirely different, each planet is to be treated of 

 apart; and therefore to begin with Mercury : It is certain that, according to 

 late and accurate observations, the ascending node of this planet, in this cen- 

 tury, viz. March 1691, is found near the 15° of Taurus, or rather at 16" 44' 

 from the first star of Aries ; and the opposite descending at 6' 15° 44' from the 

 same star ; the inclination of the plane of Mercury's orbit to the ecliptic, ac- 

 cording to Kepler, is 6° 54', which is nearly exact. Now it appears from the 

 most approved hypothesis, that the distance of Mercury from the sun, when in 

 the ascending node, is 31365 parts, of which the mean distance of the sun 

 from the earth is 100000; but when it is in the other node, that distance, mea- 

 sured in the same parts, is 45308 : the sun, when opposite to the ascending 

 node, is distant from the earth, in conjunction therewith, 98955 parts ; but in 

 the other node, the same distance becomes IOIOO7; and therefore Mercury, 

 in conjunction with the sun at the ascending node, is distant from the earth 

 67591 parts, but at the descending node 55699 ; which widely differing from 



* Vol. II. p. 591, of this abridgment. f Ibid. p. 605. 



