Over the past twenty years experiments have been conducted on weather modification, particularly on the  effects of seeding clouds with such materials as silver iodide crystals. The results are limited. Under suitable circumstances it has been possible to augment precipitation by ten to twenty percent, and to reduce the frequency of fire-producing lightning strokes. Effects on hail production have been noted, sometimes suppression and sometimes augmentation.  These results probably would be of only passing interest were it not for the potential importance to mankind of further progress in this field. Perhaps the most significant result of the experiments to date has been to bring about a change in attitude from one of skepticism to one of cautious optimism.