1884.] Level of the Cape Transit Circle. 9 



tion or at sucli a depth, below tlie surface that the heat of summer and 

 the cold of winter take four months to reach it. 



Unfortunately no deep-sunk thermometers have ever been observed 

 here, so that I am unable to say what this depth may be. 



From observations at Edinburgh it has been found that the tem- 

 perature at a depth of 12-8 feet follows the surface temperature by 2f 

 months and at a depth of 25-6 feet by o|- months. The thermometers 

 there were sunk in the rock which is porphyry. At Grreenwich, where 

 the beds were sand and flint- gravel, the numbers were, at 12*8 feet 

 2\ months, and at 25*6 feet 4^ months. 



To throw further light on these changes I have examined the 

 records of the level errors of the old transit, which is situated 54 feet 

 to the west of the transit- circle. The changes of level in this case are 

 smaller, and not very easy to follow. Sudden jumps are by no means 

 rare, and there are often long intervals without any determination, 

 but on the whole they follow a law somewhat similar to that of the 

 transit-circle levels. 



Up to 1862 the changes are much smaller than in any of the subse- 

 quent years. Now, the embankment for the Wellington Bailway was 

 completed to Salt Eiver at the end of 1860, and it has been said that 

 the effect of it has been to cause an accumulation of water of much 

 greater extent and longer duration to the north and east of the 

 Observatory than was formerly the case. It may be, therefore, that 

 the present large changes of level are due to the action of heat on the 

 water which has forced its way into crevices and hollows in the hill. 



On figure 2 curves are drawn showing the changes of Nadir-point 

 reading and of azimuth in the course of the year. The Nadir-point 

 curve agrees fairly with the temperature curve, but there is a well- 

 marked effect of rainfall in June and July. 



