President's Address lvii 
and more especially that of nutation, it still remains to be seen whether 
the last refinement of existing determinations is not to a considerable 
extent vitiated by change of latitude which was unsuspected at the 
time when the values were deduced. 
The remaining weak links of the whole chain of astronomical con- 
stants which are capable of direct measurement are : 
1. The figure of the earth. 
2. The constant of aberration. 
3. The solar parallax. 
4. The mass of the moon. 
1. Figure of the Earth.—Towards this problem a noteworthy contribu- 
tion has been made from South Africa by the completion of its geodetic 
survey. The work is only just finished—an account of it will soon be 
published ; and Colonel Morris, R.E., whose indefatigable zeal I have 
had occasion to mention more than once, has left our shores to rejoin 
the corps whose reputation he has so well sustained during his ten 
years of active labour in this colony and Natal. The work that he has 
done will be of permanent value to science, and of infinite practical use 
to the Colony. 
~ 2. Constant of Aberration.—I have already dwelt on the reasons which 
require that this determination should be repeated, and the fact that 
observations for the purpose have been in progress at the observatory 
for the past eighteen months. 
3. Zhe Solar Purallax.—Thirteen years ago I gave from this chair a 
history of our knowledge of this determination, and the principles of 
the various methods available for the purpose. Since then we have had 
another transit of Venus in 1882, and a great deal of further evidence 
on the whole subject. As the result of all this, I find no occasion to 
modify in a single phrase the opinions which I then expressed. The 
transits of Venus have proved to be among the most costly and least 
satisfactory of all the trigonometrical methods. The collateral results 
which they have furnished in the way of determining accurately many 
important geographical positions, in stimulating the invention of 
methods and instruments, must be regarded as far more valuable than 
the results they have afforded or are capable of affording towards the 
determination of the solar parallax. 
The opinion which I advanced in my former address, viz., that 
observations of minor planets afford the best trigonometrical method of 
determining the solar parallax, has been more than confirmed, and I 
would add now, with further experience, not only the best method, but 
the only trigonometrical method which, when properly executed, is 
free from suspicion of systematic error. 
