… should be still Plucking the grass to know where sits the wind. (17-18)
The wordstillmeans always. If someone plucks a blade of grass and holds it up in the air, the blade bends to indicate the direction of the wind. Merchant ships were sailing vessels that depended upon favorable winds for them to reach safe harbors in time and to return safely to their home ports. Like Salerio, Solanio is speaking metaphorically. He is suggesting that every moment he would be in a state of constant worry if he had so much invested in a merchantenterprise.
Salerio agrees and adds that every little common occurrence would cause him to think about his ships. For example, if he were to blow on his soup to cool it, his breath would make him think of a wind blowing on one of his ships. So, he concludes that Antonio must be sad because he is worried about his ships and the merchandise they contain.
However, Antonio tells them that his business ventures are not the cause of his sadness. He explains that he has several ships traveling to and from various places so that even if he were to lose one, he could still be financiallysuccessful.