There was observation - see his paper - <a href="http:&#x2F;&#x2F;calteches.library.caltech.edu&#x2F;1801&#x2F;1&#x2F;zwicky.pdf" rel="nofollow">http:&#x2F;&#x2F;calteches.library.caltech.edu&#x2F;1801&#x2F;1&#x2F;zwicky.pdf</a> - they saw the particles streak up out of the atmosphere, and measured their speeds.