There is a temperature gradient in the distilling head, hot near the pot and cooler toward the top, so the thermometer must be positioned so that it equilibrates to the vapor that is at the boiling point of the liquid.
(the thermometer; must be positioned; ) (it; equilibrates; to the vapor) (the vapor; is; at the boiling point of the liquid)
the thermometer it the vapor
must be positioned equilibrates is
to the vapor
at the boiling point of the liquid