Of the 24 Colby graduates currently working as Peace Corps volunteers, four are in the West African country Niger. Elisabeth Carpenter '95, Kristen Drake '96, Annie Clougherty '94 and Liz Ward '97 are involved in environmental education, nutrition and agricultural practices in villages throughout the country. Each of the four speaks the indigenous language of the region where she works.     Since 1961, 221 Colby alumni have worked for the Peace Corps, an average of six per year. Only 18 small colleges have produced more Peace Corps volunteers than Colby, according to Peace Corps director Mark Gearan.