In the past two years, millions of Americans have suddenly embraced the bicycle as if it were a startling invention. Annual bike sales doubled between 1960 and 1970, and there are nearly 70 million bikes in the United States today. That's more than two for every three automobiles. Of course, the bike has been around for more than 150 years, and this isn't America' s first bicycle boom. A wave of bike enthusiasm swept the land in the late 1800s and bicycle production hit two million units in 1897. Then with the coming of the auto, bicycling declined, and for decades remained popular only with children and a few adult faddists. Now, national preoccupation with air pollution and physical fitness had brought the bike back to the forefront, particularly with adults. More than eight million bikes were sold in the Untied States last year and a third of them went to adults. The year before, only 15 percent of new bike sales were for adults. The reader can also conclude that Americans are .