"You are a mean lad, to tease an old man like me!" they heard, in Caspar Potts's quavering tones. "Why cannot you go away and leave me alone?" "Don't you call me mean!" came in Nat Poole's voice. "I'll do what I please, and you can't stop me!" "I want you to leave me alone," reiterated the old professor. "I will--when I am done with you. How do you like that, old man?" And then Nat Poole gave a brutal laugh. "Oh! oh! Don't smother me!" spluttered Caspar Potts. "Please leave me alone! You have ruined my clothes!" "I wonder what's up?" said Dave to his chums, and ran through the barn to the rear. There he beheld Caspar Potts in a corner. In front of him stood Nat Poole, holding a big garden syringe in his hands. The syringe had been filled with a preparation for spraying peach trees, and the son of the money-lender had discharged the chalk-like fluid all over the aged professor. "Nat Poole, what are you up to!" cried Dave, indignantly, and, leaping forward, he caught the other youth by the shoulder and whirled him around. "You let Professor Potts alone!" "Dave!" cried the professor, and his voice showed his joy. "Oh, I am glad you came. That young man has been teasing me for over a quarter of an hour, and he just covered me with that spray for the peach-tree scale." "What do you mean by doing such a thing?" demanded Dave. "Give me that syringe." And he wrenched the article from the other youth's grasp. He looked so determined that Nat became alarmed and backed away several feet. "Don't you--you--er--hit me!" cried the money-lender's son. "What a mean piece of business," observed Roger, as he came up, followed by Phil. "Nat, you ought to be ashamed of yourself!" "Oh, you shut up!" grumbled Nat, not knowing what else to say. "I always thought you were a first-class coward," put in Phil. "Now I am sure of it." "This is none of your affair, Phil Lawrence!" "I should think it was the affair of any person who wanted to see fair play," answered the shipowner's son. "Nat, you take your handkerchief and wipe off Mr. Potts's clothes," said Dave, sternly. "Eh?" queried the money-lender's son in dismay. "You heard what I said. Go and do it, and be quick about it." "I--er--I don't have to." "Yes, you do. If you don't----" Dave ended by walking over to a barrel and filling the syringe with the spraying fluid. "Hi! don't you douse me with that!" yelled the other youth in alarm. Then he started to run away, but the senator's son caught him by one arm and Phil caught him by the other. "You've got no right to hold me!" "Well, we'll take the right," said Roger, calmly. "Now, Nat, do as Dave told you." There was no help for it, and with very bad grace the money-lender's son drew from his pocket a silk handkerchief and removed what he could of the fluid from Caspar Potts's clothing. Many spots remained. "I am afraid the suit is ruined," said the aged professor, sorrowfully. "Anyway, it will need a thorough cleaning."