What's the difference between thereat and whereat?

Thereat

Definition:

• (adv.) At that place; there.
• (adv.) At that occurrence or event; on that account.

Example Sentences:

• (1) Factors and methods being complementary and such having an effect of contrary sence are thereat especially described.

Whereat

Definition:

• (adv.) At which; upon which; whereupon; -- used relatively.
• (adv.) At what; -- used interrogatively; as, whereat are you offended?

Example Sentences:

• (1) ), whereat measurements can be taken either using cylindrical graphitic walls or "no walls".