W. Harkness—The Solar Parallax. 889 
Iunar Inequality of the Earth.—F rom observations at Green- 
wich, Paris and Koenigsberg, made during the periods stated, 
LeVerrier found the following values for the lunar equation of 
the earth: (OPM, iv, 100) 
Greenwich. --_-. ...- 1816-26 L=6'-45 
yon Tura tren sty ant er? 1827-50 6°56 
Paris osiedsowinlse 1804-14 6°61 
$0 CC ea ee 1815-45 6°47 
Kenigsberg _-_------ 1814-30 6°43 
The mean is 6’°50+0/023. 
Professor Newcomb found the following additional values: 
(WOb, 1865, App. IT, pp. 25 and 26) 
Greenwich __-.--. 1851-64. L=6"56+0"'04 
Washington ---- -- 1861-65 6°51 +0°07 
With these values of L, equation (14) furnishes the following 
values of the solar parallax : 
Moon’s Mass. do aT oy ay 
L = 6"°50 8”"-664 8"-770 87-878 8"°985 
6°51 678 "784 892 8-999 
6°56 8°744 8°851 8-960 9°068 
It would seem that the observed value of L should be quite 
free from systematic errors, because it depends upon observa- 
tions of the sun which are always made in the same way. The 
relation subsisting between small changes in the parallax, the 
mass of the moon, and the earth’s lunar inequality, are given 
by the equation 
dp =136 dL. + 0°107 (5) (25) 
It will be difficult to determine the true value of L within 
+002, and at present the uncertainty in the reciprocal of the 
moon’s mass is at least +0°%. With these data the probable 
error of p comes out +0/’-06. 
Photo-tachymetrical Methods. 
Theory.—The photo-tachymetrical methods are quite recent, 
having come into existence about 1850, when Fizeau and 
Foucault made their inventions for measuring the velocity with 
which light traverses moderate distances upon the surface of the 
earth. From the velocity of light thus obtained the solar 
parallax may be found by two essentially different methods, to 
wit: 
1st. Deriving from the eclipses of Jupiter’s satellites the time 
Occupied by light in traversing the mean distance between the 
