•with Remarks on terrestrial Befraction. 215 



ous range, running east or west, overhangs a champaign di- 

 strict, the effect of the unequal attiaction wili be at its maxi- 

 mum in the direction of the meridian. Hence the refraction 

 observed at the side of the ridge will be greatly in excess, and 

 that from the plain will be sensibly in defect. Half their sum 

 will not indicate any thing particularly anomalous ; but in as- 

 signing it to the angles of elevation and depression in equal 

 quantities, great errors are necessarily committed. Supposing 

 the stations on the fronting sides of two similar and parallel 

 ranges, both the refractions will be in excess ; yet, being nearly 

 equal, will not vitiate the corrections. The stations having 

 the same meridian, an easy method of determining the sum of 

 the refractions naturally suggests itself. Call one station A, 

 the other B, and a culminating highly elevated star S. The 

 difference between 180° and the sum of the observed vertical 

 angles SAB and S B A (allowance being made for the astrono- 

 mical refraction) will be equal to the correct sum of the refrac- 

 tions : and the excess of the other estimate will be equal to the 

 sum of the disturbances of the plumb line at the two stations. 

 Without having made any experiments of this description, I 

 have hit upon the method of properly estimating the refi-action 

 due to each observation, and proving, with various success, 

 that unequal attraction may materially affect the refraction in 

 terms of the arc. In attempting to account for the uniform 

 yet remarkably small refractions at Great Almias cliff*, it has 

 been seen, that it was assigned to the unusually high tempera- 

 ture of the air above the rock ; but as the cliff" is the abrupt 

 termination of the mountains of Wharfdale, the excess of mat- 

 ter would be entirel}', and in an important degree, in the di- 

 rection of tlie stations observed. To conclude: Will the at- 

 mosphere at equal elevations be of the same density over the 

 sea as when resting upon a huge mountain? 



In summer the observer is frequently annoyed during the 

 heat of the day by a tremulous motion in the air. Its effects 

 are sometimes visible within a few yards, giving to the objects 

 around the most bizarre appearance imaginable. When the 

 undulations are vertical, the angles will remain correct within 

 a tew seconds; but should they assume the form of a chain, 

 when shaken, great errors may be committed. I have already 

 given two instances of irregular refraction at Symon Seat, and 

 may now add other two from the same station. Sept. 10, 1 822, 

 (the thermometer having a range of only one degree in the 

 course of four liours,) the tremulous motion being confined to 

 the south-east, Jack-hill and Great Almias, both in that direc- 

 tion, were observed to be extremely distorted : their observed 



depressions 



