Sturdy steel body to withstand rough handling, with three low friction balls bearing wheels and aligned axles to ensure their rolling along a true line without deviation to provide accurate data. Each cart weighs about 1.5 kg. The cart bed is covered with a rubber pad and the ends of the cart rise above the bed to carry a brick or any other mass. A clip is provided at one end of the cart to attach a tape. The pins of each cart fit with holes on the other so that they can be stacked to double the mass.