You will have noticed that in order to play the low octave of the C major scale, it was necessary to produce some extra notes by bending. Also, in the high octave, the note B was missing. This is because of the way the harmonica is tuned.
It is possible to create melodies without all of the notes of the major scale. In fact, the major scale is only one of many different scales used in music. By leaving out the 4th and 7th degrees of the major scale, the major pentatonic scale is created. As the name suggests, pentatonic scales contain only five different notes. There are many melodies, particularly in folk and gospel music which are derived from the major pentatonic scale. One you have already learnt is Swing Low, Sweet Chariot. Others include Amazing Grace and Tom Dooley. The C major pentatonic scale is shown below.
Here is the C major pentatonic scale played in the middle octave. Once again, try mentally naming the scale degrees as you play.
Once you are comfortable playing the major pentatonic scale in the middle octave, try playing it over the full 3 octave range of the harmonica. As with the C major scale, you will need to bend hole 3 to obtain the 6th degree of the scale (an A note) in the low octave.