AUGUST. 
281 
but nothing is more common than to meet with them in 
the pupa, sometimes hard, rounds 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 -fields 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 
