Astronomer <a href="https://en.wikipedia.org/wiki/Angelo_Secchi" id="mwdg" rel="mw:WikiLink" title="Angelo Secchi">Angelo Secchi</a> demonstrates the <a href="https://en.wikipedia.org/wiki/Secchi_disk" id="mwdw" rel="mw:WikiLink" title="Secchi disk">Secchi disk</a>, which measures water clarity, aboard <a href="https://en.wikipedia.org/wiki/Pope_Pius_IX" id="mweA" rel="mw:WikiLink" title="Pope Pius IX">Pope Pius IX</a>'s yacht, the <i id="mweQ">L'Immaculata Concezion</i>.