Sentence:  It is only by considering the distance to which large quantities of these commodities are carried, ... that we can form any idea of the magnitude of the trade with India by land, and are led to perceive, that in a <WSD>Disquisition</WSD> concerning the various modes of conducting this commerce, it is well entitled to the attention which I have bestowed in endeavouring to trace it. 
Meaning of the word marked with <WSD> tags:  A lengthy, formal discourse that analyses or explains some topic;  a dissertation or treatise.