She told him about an important difference between the irrotational wave he had just described (of a ideal liquid with a free surface) and a rotational wave which Gerstner had described a long time before (the cycloidal wave).
Siya na sinasabi sa kanya tungkol sa isang mahalagang kaibahan sa pagitan ng irrotational ikaway niya lang na inilalarawan (ng isang ideal na likido sa isang libreng ibabaw) at isang palitin iwagayway na Gerstner ay inilarawan ng isang mahabang oras bago (ang cycloidal kaway).