More than 100 000 cosmic-ray tracks from extensive air showers were scanned in pictures of cloud chambers after the manner of McCusker. In addition, positive- and negative-ion columns were separated by an electric field, and individual droplets were counted; histograms of numbers of tracks versus droplet density are shown. Artificially produced low-droplet-count tracks have been used to measure the scanning efficiency for quarklike tracks. To date the upper limit for the $\frac{2}{3}e$ quark flux is 3 × ${10}^{-11\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$, and the limit for $\frac{1}{3}e$ particles is 3 × ${10}^{-10\mathrm{}}$ ${\mathrm{cm}}^{-2\mathrm{}}$ ${\sec }^{-1\mathrm{}}$ ${\mathrm{sr}}^{-1\mathrm{}}$.

• Received 3 May 1971

DOI:https://doi.org/10.1103/PhysRevLett.27.51