VOL. LVIIl.] PHILOSOPHICAL TRANSACTIONS. 539 



the menstrual parallax are worthy of consideration ; and that nothing has been 

 yet executed, by which it has received a determination sufficiently accurate; 

 for, in regard to observations on the sun, the whole quantity is too small to be 

 minutely observed in right ascension : and with respect to the application to 

 Mars and Venus, as suggested by Dr. Gregory, Mr. S. does not know that any 

 thing has been done ; and indeed no wonder, as the theory of the motion of 

 Mars and Venus has not been as yet so critically reduced to computation, as to 

 render their parallaxes (though in themselves much greater) deducible with equal 

 certainty as that of the sun. 



What Mr. S. therefore has now to propose, is a method of observing the men- 

 strual parallaxes of Mars and Venus, without laying any undue stress on the 

 theory of their motions. The first opportunity of making an observation for 

 this purpose, will be at the next opposition of Mars ; which according to the 

 nautical almanack, will happen the 26th of October next, in the morning. 

 He therefore endeavours to illustrate this matter by taking that as an example. 

 The distance of Mars from the earth will then be somewhat less than the mean 

 distance, that is, as 1 to 2. 2 ; and consequently his double n)enstrual parallax, 

 according to Mr. Maskelyne, will be near 31" in the point of opposition. Now, 

 as the moon will be at full, not above 12 hours preceding that opposition, the 

 moon will be nearly in the most favourable situation for the purpose. 



For this end, let an accurate observation be made upon the place of Mars at 

 the following times, viz. first, near the time of the new moon, preceding 

 Mars's opposition ; or more properly at the nearest opportunity, to the time of 

 the moon's opposition to Mars; which will happen in the night between the 12th 

 and 13th of October : 2dly, let the place of Mars be observed when the moon is 

 nearest her quartile with Mars ; that is, between the IQth and 20th of the same 

 month: 3dly, let an observation on Mars be made when the moon is in con- 

 junction with Mars, the nearest to his opposition with the sun ; that is, between 

 the 25th and 26th of ditto : 4thly, let Mars again be observed when the moon 

 has moved on to her quartile with Mars, viz. between the 3 1 st of October, and 

 1st of November : and 5thly and lastly, let the place of Mars be observed, when 

 the moon has again got to her opposition with Mars, which happens between the 

 7 th and 8 th of November. 



Now it is manifest, that when the moon is in conjunction or opposition to 

 Mars, the centre of the earth, the centre of Mars, and the common centre of 

 gravity of the earth and Mars, will be nearly in a right line, and consequently, 

 that an observer will then see Mars, in the same place in the heavens, as if 

 the common centre of gravity was the same as the centre of the earth ; there- 

 fore then the place of Mars will be unaffected by a menstrual parallax ; and such 

 will be the 1st, 3d, and 5th, of the observations above f)ropounded. 



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