What is a set with no elements called in mathematics?

A series of travel guides gives its title to which 1985 Anne Tyler novel? A service line, an attack line, a net and a ball are all features of which sport? What is a set of buildings housing soldiers in a garrison called? What is a set of draft animals harnessed together called? What word names a setback or piece of misfortune, such as a military defeat? A sexagesimal number system is based on which number? A sextant is used by navigators most simply to determine what? A sextant was among the tools that which duo used on their journey of about 8,000 miles to the Pacific Northwest and ba…