Harpsichord notation

[pies]

Anyone know what 4', 8^i, 8^s and other similar symbols mean?

[quantum]

4' would mean "4 foot" referring to a stop sounding one octave higher than written. 

My guess is the 8^i, 8^s are "8 foot" stops and sounds as written.  The terminology of footages is borrowed from organ nomenclature. 

If a harpsichord has more than one 8' stop, my guess is that 8^i, 8^s differentiates between these stops.  There is also a thing called a lute stop, not sure how this is indicated. 

I play organ but not harpsichord - thus my knowledge comes more from the organ field. 

Somewhat related:
I've got a free harpsichord sampled soundfont that I play with from time to time here:
https://sonimusicae.free.fr/accueil-en.html
If you have a MIDI keyboard you can play around with it and see what the different stops and footages sound like.