It has been found that the production curves (counting rate against thickness of lead) of electron-produced and of photon-produced showers are similar and that the average penetrating power of the rays from electron-produced and from photon-produced showers is the same. Shower production curves have been obtained at four elevations and analysis of the curves shows that the absorption per nucleus of the shower-producing radiation (photons) is approximately proportional to the square of the atomic number. Coefficients for lead (0.33 ${\mathrm{cm}}^{\ensuremath{-}1}$), iron, and air have been determined, and a comparison with theory is given. Also the penetrating power of the shower rays emerging from a block of lead has been measured and found to be independent of the elevation and of the thickness of the lead from which the rays emerge.