Ivi The Transactions of the South African Philosophical Society 
should not be left in doubt, and therefore it is desirable that determina- 
tions of the constant of aberration should be made in both hemispheres, 
so as to eliminate the effect of a possible sensible value of A. Sucha 
determination of the constant of aberration must be regarded as one 
of the most pressing needs of exact astronomy in the present day. 
Accordingly, for the past eighteen months, the work has been in pro- 
cress at the Cape Observatory, followed out on the second of the 
previously mentioned methods. This plan affords not only a determina- 
tion of the constant of aberration, but a record of the variation of 
latitude, which for the future must be rigorously measured and taken 
into account in the reduction of all observations of the absolute position 
of celestial objects. 
In this review of the problem, I have endeavoured to show not only 
the intimate correlation of our problem with the whole basis of 
astronomy of precision, but also to point out the weak links in the 
chain of our data, and to find the best means of strengthening them. 
I have hitherto left all but unmentioned the determination of the 
masses of the planets. When a planet has a satellite or satellites, this 
problem is best solved by determining the mass from observations of 
the satellite, and by computing from these observations the period and 
mean radius of the orbits, and hence the attractive force of the planet, 
on the same principle as we have seen that the mass of the sun was 
determined by the distance which it attracted the earth towards itself 
in a unit of time. The discovery of the satellites of Mars has enabled 
the mass of that planet to be determined with far greater precision than 
was previously possible ; but observations of these faint objects are re- 
served for telescopes far larger than any we can boast at the Cape. The 
same may be said of observations of the satellites of Saturn, Uranus, 
and Neptune; but the work has been adequately taken in hand at 
Pulkova and Washington. In 1891 a long series of observations of the 
satellites of Jupiter was made here, which it is hoped will contribute 
towards the accurate knowledge of the mass of that giant planet. The 
masses cf Mercury and Venus cannot be determined as independent 
problems ; they can only be arrived at by the effect of their attractions 
on other planets and on each other, and their ultimate values must 
depend on a complete discussion of the whole dynamical problem of the 
solar system. 
The value of the constant of precession will be continually better 
known as time goes on, from the daily routine operations of fixed 
observatories ; and the same may be said of the constant of nutation, 
although the latter requires more refined and special methods, such as 
those which have been so long and successfully followed with the prime 
vertical transit at Pulkova, But with regard to both these constants, 
