AUGUST. 281 



but nothing is more common than to meet with them in 

 the pupa, sometimes hard, round, and substantial. In the 

 pupa state of moths having pectinate antennae, we may often 

 discover the sex by examining the form of these organs, 

 which is easily done by the lines and wrinkles of the pupa : 

 the males have them much broader and larger than the 

 females ; the shape and size of the abdomen, too, is often a 

 sufficient distinction while in this state. 



C. I was much pleased lately at discovering a fine 

 articulate echo in our orchard -field, near the first of the large 

 elms in the road. It repeated five syllables with distinctness 

 by day, and probably in the still calm evening would repeat 

 more. 



F. Echoes formerly were subjects of much wonder and 

 admiration : many fanciful and poetical theories were made 

 by the ancients to explain them : they are now, however, 

 well understood. Sound consists of undulations or waves in 

 the air, diffused in every direction from the producing cause, 

 as the circles on smooth water are spread from a stone 

 dropped into it. It is also capable of being driven back on 

 meeting with any impenetrable body, as a ball rebounds 

 when thrown against a wall. All that is necessary to pro- 

 duce an echo is an intervening wall, or other body, at right 

 angles to the course of the sound, without any intermediate 

 object to break or destroy it. Such is the case in the one 

 you mention ; the centre or focus is on a rising hill : at the 

 distance of two or three hundred yards, is the end of a large 

 barn, exactly at right angles to the direction. The necessity 

 of this position is shown by the fact that if you go a yard to 

 the right or left, the echo is destroyed ; so it is if you go 

 higher up the hill or come lower ; in the former case, the 

 sound would be reflected lower than your position, in the 



