The hatchet or hacksaw blade planimeter, consists only of a sharpened pointer, a rigid arm, and a knife edge. Its theory was first given by F. W. Hill in thePhilosophical Magazinein 1894 and was later simplified by Satterly. Hill and Satterly showed that the numerical value of the area within a closed curve traversed in the proper manner by the pointer is equal to the product of the length of the planimeter arm and the net displacement suffered by the knife-edge as measured along an arc with the pointer at the center of curvature. The present paper gives a relatively simple derivation yielding essentially the same result but using an entirely different method of approach. It is also shown that the procedure for operating this planimeter can be simplified when determining areas within certain types of closed curves. The accuracy of the planimeter is discussed.