48 
Transactions of the Boyal Society of South Africa. 
observed phenomena. I may illustrate my point again by reference to the 
laws of Kepler. It is now well known that these laws are only rough 
approximations to the actual truth, and that though they might serve as a 
useful basis for prediction over a short interval, a few years at most would 
suffice by showing a rapidly increasing departure between the observed and 
predicted positions of the planets to indicate that these laws require 
amendment. 
That the direction this amendment should take followed so soon on the 
original discovery of Kepler's laws, was due to the genius of Newton, who 
showed that the theory of universal gravitation propounded by him not 
only adequately accounted for the laws enunciated by Kepler, and pointed 
to their imperfections, but served to co-ordinate as due to a single cause 
even more recondite phenomena such as the leading inequalities in the 
motion of the moon, the processional motion of the earth, and the 
phenomena of the tides. This theory further reduced to order those 
astronomical vagaries the comets, showing that, so long at least as they 
remained within the precincts of the solar system, their motions were 
governed by it, while observations of double stars have established beyond 
question that even remote parts of the universe are still subject to the 
same laws. 
The dynamical laws propounded by Newton, which to-day virtually 
form the basis of all astronomical prediction, enable us to trace back as 
well as to trace forward the history of the solar system, and to confront 
modern observations with historical records. Needless to say, in but rare 
instances do these records possess the necessary elements of precision to 
strengthen the existing data required by the astronomer ; but there are 
important exceptions. For instance, a very small uncertainty in the 
elements," which in conjunction with Newton's laws govern the motion 
of the moon, will produce by lapse of time a large change in the 
comparatively small area of the earth's surface over which a total eclipse 
of the sun is visible as such. Thus, a record that a particular eclipse was 
seen as total in a given locality becomes an observation of precision, 
provided only the chronological date at which the eclipse occurred can be 
traced with sufficient certainty to ensure the identification of the eclipse 
concerned. 
The confrontation of modern with historical observations of such a 
character has served to establish beyond question the high degree of 
accuracy with which the laws of Newton represent the motions within the 
solar system, and their reliability as a basis of prediction for years, 
perhaps for centuries, to come. It is, however, on various grounds quite 
certain that these laws in themselves are not absolute, far-reaching though 
they are, and that they in turn like those of Kepler must be superseded by 
laws still more exact. 
