Sentence:  But, if it does so sit, it is plain that the <WSD>pommel</WSD> must rise sufficiently to secure the withers from pressure; therefore it follows, that a horse whose withers are higher than common, (a well-built hunter for example,) requires a <WSD>pommel</WSD> higher by so much as he excels the generality of horses. 
Meaning of the word marked with <WSD> tags:  The upper front brow of a saddle.