Mr. Fitzgibbon's Plow
In chapter 4 the book explains that Mrs. Frisby knows that Mr. Fitzgibbon is going to plow his farm any day now. She knows that he plows the farm after the frost is gone. Mrs. Frisby is worried that if the plow came too soon that Timothy would not be able to move when he was worried. Then Mrs. Frisby heard a loud, sputtering roar. It was none other than Mr. Fitzgibbon starting his tractor.