McGuffey's Sixth Eclectic Reader
The compass of the voice is its range above and below this pitch. To avoid monotony in reading or speaking, the voice should rise above or fall below this keynote, but always with reference to the sense or character of that which is read or spoken. The proper natural pitch is that above and below which there is most room for variation.
To strengthen the voice and increase its compass, select a short sentence, repeat it several times in succession in as low a key as the voice can sound naturally; then rise one note higher, and practice on that key, then another, and so on, until the highest pitch of the voice has been reached. Next, reverse the process, until the lowest pitch has been reached.