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We report on the detection of dark matter in the cluster Abell 2218 using theweak gravitational distortion of background galaxies. We find a highlysignificant, coherent detection of the distortion in the images of thebackground galaxies. The inferred 2D mass distribution has a peak that iscoincident with the optical and X-ray centroid. The qualitative distributionsof the cluster light, the X-ray emission and the dark matter are similar andthe projected total mass, gas, and light surface densities are consistent witha $r^{-1}$ profile at distance of $r > 180^{\prime\prime}$ from the cluster cDgalaxy. Using the weak lensing technique, we determine a lower bound for thetotal mass in A2218 of $(3.9 \pm 0.7) \times 10^{14}$~h$^{-1}$~M$_\odot$ withina fiducial aperture of radius 0.4~h$^{-1}$Mpc. The associated clustermass-to-light ratio is $(440 \pm 80)$~h~$M_\odot/L_{\odot B}$. The massestimated by the weak lensing method is consistent with that inferred from theX-ray data under the assumption of hydrostatic equilibrium and we derive anupper bound for the gas-to-total mass ratio at 400~h$^{-1}$kpc of$M_{gas}/M_{tot} = (0.04 \pm 0.02)$h$^{-3/2}$.Comment: 25 pages, Latex, no figures. Complete version with figures available  by anon ftp at ftp://ftp.cita.utoronto.ca/ftp/cita/squires/a221

The Dark Matter, Gas, and Galaxy Distributions in Abell 2218: A Weak Gravitational Lensing and X-Ray Analysis