Securing new spindles in an old bannister rail

[Possom]

After removing the paint from an old staircase, I realised that every second spindle had at some time been removed. The spindles are a simple square in shape.

If I wanted to place new additional spindles at the original missing locations, how would a person go about securing new spindles in place?

If they were just glued, there would have to be some way to hold in place whilst setting; or would they additionally have to be nailed/screwed into the handrail/bottom rail? Or perhaps only nailed/screwed, and not glued?

Originally there were no slots cut into the bottom rail/handrail to accommodate the spindles.

 

[Tigercubrider]

Possible nailed at an angle and filled? Drill a hole first.

 

[Possom]

The spindles which remain in place and are each an equal distance apart. What I noticed is, the locations of the removed spindles are not done in this way; spaces vary.

I would guess this was done as visual thing so it helped the visual distance between the spindles if a person is walking upstairs or alternatively walking downstairs. I noticed the spacing of these removed spindles changes about halfway up.

 

[foxhole]

Visually spindles only look evenly spaced if they are . Spacing should be under 100mm for safety .
I would glue and also use panel pins to secure ( easy to punch in the heads)