Drawing, Prince Rupert's drop by [Geminiano Montanari?] Drawing of a glass drop of they type which later came to be known as 'Prince Rupert's drops', found on the last page of the document. This ‘glass bubble’ in the shape of a curved teardrop had the property that its thicker part could not be broken with a hammer, but breaking off the tip of its tapered end would shatter it completely. Subject: Trade / Glassmaking Read to the Royal Society on 6 April 1671. [1671]