input of while loop to come from output of `command`

#I used to have this, but I don't want to write to the disk
#
pcap="somefile.pcap"
tcpdump -n -r $pcap > all.txt
while read line; do  
  ARRAY[$c]="$line"
  c=$((c+1))  
done < all.txt  

The following fails to work.

# I would prefer something like...
#
pcap="somefile.pcap"
while read line; do  
  ARRAY[$c]="$line"
  c=$((c+1))  
done < $( tcpdump -n -r "$pcap" )

Too few results on Google (doesn't understand what I want to find :( ). I'd like to keep it Bourne-compatible (/bin/sh), but it doesn't have to be.