Sentence:  When a horse becomes blown, on approaching a fence, you should pause for a second, and turn him round: it will greatly relieve him, and you stand a much better chance of getting over: and indeed, it is generally preferable to turn the horse round at a <WSD>"rasper</WSD>", or large jump. 
Meaning of the word marked with <WSD> tags:  A fence that is challenging for a horse to jump over.