TOL. LVIII.] PHILOSOPHICAL TRANSACTIONS. 537 



whoever considers the great obstructions that the water of the sea meets with 

 in its motion to obey the influence of the moon; the great difficulty in 

 ascertaining the true height of the tides, from the many disturbing causes 

 intervening; and the many uncertainties, and want of coincidence, that have 

 attended, and must attend, such observations , must confess, that this matter 

 does not seem capable of such a determination from that quarter, as the present 

 state of astronomy requires. 



Accordingly, since the time of Dr. Gregory, those great astronomers 

 Dr. Bradley, De la Caille, and others, have applied themselves to determine 

 the quantity of the menstrual parallax from solar observations: but thou^ 

 these have given cause to suppose that the relative gravity of the earth and 

 moon are not above ^ of the quantity deduced from the tides; yet, as the 

 observation of these small angles principally depends on the observation of the sun'^ 

 right ascension (which, depending on the measure of time, is less capable of 

 exact observation, than if depending on divided instruments) ; the deductions 

 thence drawn seem still wanting of that certainty which the subject demands; 

 and if to this we add, from a deduction of Mr. Maskelyne, that the relative 

 gravity of the earth and moon is as 76 to 1, derived from the effect that the 

 moon produces in the nutation of the earth's axis; the relative gravity, and 

 consequently the parallaxes depending on it, will be reduced to almost one half 

 of those resulting from Sir Isaac's determination. 



It is true, that the quantity of effect of the menstrual parallaxes will not be 

 great, if computed on Mr. Maskelyne's induction, for as much as that the 

 common centre of gravity will be considerably within the earth's surface; yet, 

 even in that case, the sun's transit over the meridian, when the moon is in one 

 quadrature, will differ nearly one second of time from that observed in the 

 opposite quadrature; and though de la Caille and Mayer have formed equations 

 depending on the moon, to be applied to the equation of time; yet, if we are at 

 an uncertainty, whether the maximum of this equation is one second, two-thirds 

 of a second, or half a second of time, each way, we are still under a material 

 difficulty; for though these differences are so small, that it is not easy to deter- 

 mine them exactly from solar observations; yet, as they are capable of creating 

 a sensible difference in these observations, they will, so long as they remain 

 undetermined, prevent that solidity and firmness to the solar observations, which 

 is the more necessary as they are the foundation of all the rest: but with respect 

 to those planets, that in their periods come nearer to us than we to the sun, the 

 observations on them will be affected by a greater uncertainty. 



The determination of the menstrual parallax is of still more importance, 

 as it is a necessary consideration in the determination of the sun's parallax ; and 

 this, whether deduced from Mars or Venus, as shall presently be shown more 



VOL. XII. 8 Z 



