1 854. ] On the Mirage of India. 1 69 



pi. IX. is of this character ; the reflecting medium lying below the 

 object and the spectator's eye. But in the case of the Mhow church, 

 the phenomenon is aided by an elevation of the object above the 

 intervening surface of the earth. 



Thus far had I written in Huzara, where I had no opportunity of 

 reference to books. Since my arrival in Calcutta I have referred to 

 Brewster's treatise on mirage. He seems there to attribute the kind 

 first noted by me to the reflection of the image from a denser stratum 

 of atmosphere ; although he is treating of observations made at sea. 

 This is I think a mistake. The reflection of the sun's rays from the 

 surface of the ocean can scarcely be sufficient to heat the atmosphere 

 in contact to such a degree as to cause a perceptible deficiency in its 

 density below that of the incumbent strata : and, were it so, the stratum 

 thus rarified would immediately ascend. It is undoubtedly a stratum 

 of vapour which forms the mirror, and its presence in that position is 

 thus to be accounted for. — 



At night, the mist, parting with its caloric, becomes specifically 

 heavier than the atmosphere, and settles on the earth. There on clear 

 nights the radiation of the caloric from the mist to the vault of heaven, 

 precipitates it in dew upon the earth. Again when the sun rises, the 

 earth's surface imbibes the rays and the dew is evolved in vapour which 

 at first is transparent. 



This vapour being of rather less specific gravity than the lowest 

 stratum of air, rises above it, until it meets with a stratum somewhat 

 elevated, which the reflected heat from the earth's surface has not 

 tempered. To this stratum it parts with a portion of its caloric until 

 its rarity is so much abated that it cannot ascend higher ; and it then 

 hangs like a canopy in the air, continually increasing by additions of 

 vapour from beneath, but as continually decreased by the escape of 

 particles above. Accordingly the phenomenon is only or chiefly 

 observable from the 1st to the 2nd or 3rd hour after sunrise and when 

 the nights are rather chilly and the skies clear. 



Brewster mentions the reflected image (in the atmosphere) of a 

 ship and of the ship's shadow or image in the water. This I presume 

 could be exhibited only from long distances and when the illumination 

 is very strong. I have never observed it, 



z 2 



