SOUND. 117 



ESTHER. 



And also as in that of the syrup and water; for the transpa- 

 rency of the air, in a hot day, is much affected, and every ob- 

 ject appears, as it were, in motion. 



MRS. F. 



At midnight, on the contrary, the air is transparent and of 

 a uniform density, and more fit to transmit sounds to the ear 

 without any interruption. 



ESTHER. 



The syrup and water is a good illustration of the mirage of 

 the desert. 



MRS. F. 



Yes; but, I presume that you are all well acquainted with 

 this phenomenon and its causes, and, as I have already ob- 

 served, the points of resemblance between the nature of light 

 and sound are numerous; but, I was telling you that falling 

 or new-fallen snow obstructs sound; the very opposite effect 

 is produced by hardened snow, water, or ice. Of this, I can 

 give you some curious examples. 



HENRIETTA. 



Pray do. 



MRS. F. 



Lieutenant Forster conversed with a man across the ice of 

 Port Bowen harbour, a distance of about a mile and a quar- 

 ter; and Major Denham gives his authority, that the human 

 voice was heard at Gibraltar, at a distance of ten miles. 

 When the ground is dry and hard, or rests upon a continuous 

 stratum of rock, sound is propagated to a much greater dis- 

 tance: hence the practice in many countries of ascertaining 

 the approach of horsemen by applying the ear to the ground. 

 The sound of cannon has been heard at distances of 120 to 

 200 miles. 



MARV. 

 Oh! mamma, that is far indeed; when did it happen 1 ? 



