538 PHILOSOPHICAL TRANSACTIONS. [aNNO 1 768. 



particularly; but first to state the quantity of the menstrual parallax, according to 

 the best data yet known, by a contrary process; and, taking the mean quantity 

 of the sun's parallax, according to the determination of Mr. Short, at 8".8, 

 and the relative gravities of the earth and moon, according to Mr. Maskelyne, 

 as 76 to 1, and the mean distance of their centres equal to 60-J- semidiameters; 

 we shall then have the distance of the earth's centre from the centre of gravity, 

 at -^ of the earth's semidiameter (that is, -i- of that semidiameter within the 

 earth's surface) and the menstrual parallax equal to ^Sj. of the sun's parallax; 

 consequently about 7"; and the double menstrual parallax, or vacillation, arising 

 from the whole diameter of the epicycle, 14"; the mean menstrual parallax of 

 Mars in opposition 2Q"-^; the greatest 38"-J^; and that of Venus 4Q"; hence it 

 follows, that, were a person to attempt the sun's parallax, by the diurnal motion 

 of the earth, applied as a basis to Mars in opposition, as has formerly been tried; 

 and should the moon be at new or full at the same time, the change of place 

 of the earth's centre, in its own epicycle, woulil amount to an angle seen from 

 Mars of 1".3 nearly; that is, in case the interval between the observations was 

 8 hours, and Mars at his mean distance; but if Mars was not at his nearest 

 distance, this change would in the same time amount to 1".7 nearly. In like 

 manner if a transit of Venus happen near the new or full moon, as will be the 

 case next year, the time of the transit will be affected by a change of place, such 

 as the earth's centre will describe in its epicycle, during the time of the whole 

 transit, if the beginning and end are observed in the same place ; or during the 

 difference of absolute time, at which the transit appears to begin or end to 

 different observers in distant meridians. Thus, when the same observer sees the 

 beginning and end in the same place, the base described by that observer, from 

 the earth's diurnal motion, must be corrected by the space described by the 

 earth's centre, in the circumference of its epicycle, during that time ; which, if 

 it be supposed of 7 hours, will amount to an angle of I'.Q, seen from Venus; 

 but, where the beginning or end is seen by different observers in distant meri- 

 dians, as the difference of absolute time can hardly amount to above 15 minutes 

 the change of place of the earth's centre will for that time be but small ; how- 

 ever, at the rate beforementioned, it will for 1 5 minutes affect the parallactic 

 angle seen from Venus, by about -pl-j-, of a second ; and the parallax of the 

 sun, by about ,^-5-0- part of the whole : but this proportional part will remain the 

 same, whether the distance of meridians be such as produces a greater or less 

 difference of absolute time than 15 minutes.* 



From what has been said, Mr. S. supposes it will appear, that the effects of 



* If an error of ^^ part of the whole may be supposed in the observation for determining the sun' 

 parallax by tlje transit of Venus, a neglect of the menstrual parallax may make it ^^ part of the 

 whole. — Orig. 



