Sentence:  The wooden upright was now standing in the middle of the floor, and the two <WSD>booms</WSD> were fitted into its grooved side and hoisted as high as hands could reach. [...] Two by two, one at each end, the students proceeded along the <WSD>boom</WSD>, hanging by their hands, monkey-wise. [...] Two by two the students somersaulted upwards on to the high <WSD>boom</WSD>, turned to a sitting position sideways, and then slowly stood up on the narrow ledge. 
Meaning of the word marked with <WSD> tags:  A gymnastics apparatus similar to a balance beam.