21i Determinalion of 0ie Altitude of Great Whcrmide -, 



probable, since the variation is known to exist in India (pro- 

 bably throughout the year), where the radiation is so conside- 

 rable, and the dews in consequence so copious. In these 

 climes the retraction, according to Captain Warren, remains 

 at its maximum during the night. Another proof may be ad- 

 duced. At an elevated window of the Observatory the varia- 

 tion, although affecting observations made on the ^^-feet stand 

 in the garden below, could not be said to have ever been per- 

 ceived. Setting aside the investigation of the cause, the errors 

 produced by the effects may be easily avoided by takmg the 

 mean of the extremes, which seldom differs many seconds 

 from the constant angle. This remark is confirmed by the 

 obser\'ations of Captain Warren as given in the ninth volume 

 of the Asiatic Researches. It is rather singular that the Cap- 

 tain should fail to draw a similar conclusion. 



The small increase of refraction occurrmg abruptly near 

 sunset, appears to be confined to situations where the preced- 

 ing variation is observable. 



\Vhen the ray x\Q.?ix\y grazes an intermediate piece of ground, 

 the angle, on a hot day especially, will be a little vitiated. 

 Happily this is of rare occurrence : three or four instances 

 would be the utmost I could quote. 



When a mountain is the object of observation, and the sum- 

 mit is not pointed, but flat or slightly rounded, it is evident 

 that the depressions will be observed in defect, and the eleva- 

 tions in excess. In the former case, the ground immediately 

 contiguous to the pole marking the station will be confounded 

 with the furthest visible part of the hill in the same direction ; 

 and in the latter, the /?;o/ of the staff" is invisible, and a nearer 

 part of the summit will be observed. Its value must in general 

 increase with the magnitude of the angles, and its effects in terms 

 of the arc diminish with the increased extent of the latter. As 

 it cannot well exceed three or four feet, it would not have been 

 alluded to, were it not to show that it gives to the refraction 

 of short arcs the appearance of being inexplicably large. Se- 

 veral of the stations adverted to in the following list of refrac- 

 tions, consisting of piles of stones, towers, walls, rocks, &c. to 

 which the above remarks are inapplicable, they are pointed 

 out as affording a fair confirmation of the theory advanced. 



Another source of irregularity in the ratios of the refi^action 

 to the arc, is the unequal disposal of attraction around the ob- 

 server, which altering the situation of his zenith, increases the 

 elevations in one part of the horizon, and diminishes them in 

 another. The disturbing causes will be varied m their effects 

 nccordiiig to their arrangement around the horizon, their bulk, 

 specific gravity, distance, and elevation. When a mountain- 

 ous 



