One key question over the South Sea Bubble of 1720 has been whether or not it was a ‘rational’ bubble as economists would now define it. The underlying analytic dichotomy between 'rational' and 'irrational' bubble seems to have hit a point of diminishing return, however. We need fresh studies that combine different sources and methods. This paper contributes to this larger task by focusing on one of the largest investors during the Bubble period: James Brydges the Duke of Chandos (1674-1744). The case is unique in that we have both financial accounts and the letters he sent to his friends and financial partners in 1720. Combining these sources, the paper reports the duke's investment strategies, and reveals how he was able to coordinate a large-scale speculation without risking his reputation and respectability. Strikingly, however, while pursuing a rational strategy to 'ride the bubble', Brydges viewed other actors to be driven by folly, greed and irraitionality. This, combined with other strategies, enabled him to maintain an impression of social distance from the very act of speculation. It will be shown that sophisticated market speculation - characteristic of modern financial market - was undertaken in ways that were consistent with the early modern moral economy.