Sentence:  Dig your clay with a ſpade in <WSD>ſpits</WSD> of ordinary bricks; dig two, three, eight, ten or twenty loads of clay, more or leſs as you pleaſe; [...] then take theſe <WSD>ſpits</WSD> of clay, after they are tried in the ſun, ſurround your pile of wood with them, [...] 
Meaning of the word marked with <WSD> tags:  The amount of soil that a spade holds; a spadeful.