A WATERFALL ten times the height of Niagara, and a tree 200 feet high and 4,000 years old-these arc the sort of things one encounters in the Yosemito Valley in California, of which, as of the better known parts of the State-San Francisco, San Diego and Los Angeles -Miss Rachel Humphreys will talk today. As a practised travel-writer whose books include ‘Travels East of Suez ' and ' Algeria, the Sahara and the Nile,' Miss Humphreys knows how to do justice to her theme.