1888.] President's Address, xxvii 



most scrupulous accuracy and with an energy that fcAv men could 

 maintain under the difficult circumstances of such a work. It has 

 now been going on for five years : a chain of triangles has been 

 carried from Newcastle through Natal and into the Colony as far as 

 Port Elizabeth, and base lines were measured at Pietermaritzburg and 

 Port Elizabeth. Everything, it must be remembered, in such work 

 depends on the accuracy with which the base (or starting) line is 

 measured, and the exact co-efficients of expansion of the bars used 

 have yet to be determined ; but still, with the preliminary co-efficients 

 adopted, the length of the Port Elizabeth base calculated from the 

 Pietermarit burg base carried through the 50 triangles over a distance 

 of 600 miles differs from the measured length by less than two inches 

 and small corrections have yet to be applied which will probably 

 diminish this discordance. It will perhaps give a better idea of the 

 accuracy attained if I put it in this way : to reconcile the two bases 

 exactly it is necessary to correct each of the angles of the triangles 

 by 1-10 of a second of arc, a quantity which we cannot measure with 

 certainty with instruments of the present day. 



Truly, S. Africa should be proud of having accomplished such a 

 work and I wish it were more generally appreciated at its true value. 

 When completed it will be an important geodetic measurement, second 

 to none in accuracy, and of lasting benefit to the country in supplying 

 exact reference points for future surveys : we shall then be able to 

 say with truth that S. Africa is the best surveyed colony in the world. 



Considerable progress has been made in our knowledge of the tides 

 by Mr. Ferrel, Sir W. Thomson and Prof. Darwin. The height of 

 the tide is legarded as being the algebraic sum of a number 

 of tides due to various theoretical causes : by means of harmonic 

 analysis the constants of these tides are determined from the records 

 at any port, and then Sir W. Thomson's tide-predicter enables us 

 from these constants to predict the tides for any year at the particular 

 station with accuracy. Self-recording tide gauges have for several 

 years been at work at Cape Town, Port Elizabeth and East London, 

 ^nd also self-recording anemometers. They were established at the 

 suggestion of Dr. Gill by the Colonial Grovernment, but most unfor- 

 tunately funds for the reduction of the records are not forthcoming. , 



THE MOON. 



In the theory of the Moon's motion there has not been much pro- 

 gress, though important memoirs have appeared dealing with special 



