In most mechanical clocks the source of energy, the weight, drives the escapement through a large step-up gear ratio; thus the impulse the escapement gives to the pendulum at each swing is small. Any changes in friction in the pivots caused by the viscosity of the oil varying with temperature have an appreciable effect on impulse. An even greater effect in turret clocks comes from the action of wind and weather on the exposed hands. Variations in impulse cause changes in the pendulum arc and lead to erratic timekeeping. To overcome this, a device known as a 'remontoire' is sometimes adopted for providing a constant force at the escape wheel arbor. Essentially, it comprises a differential gear. The train proper is connected to the input of the gear through a detent, and the output of the gear drives the escape wheel arbor. The differential cage carries a wieght on the end of an arc, and when the train input is locked the weighted arm provides the torque for driving the escape wheel. When it has dropped through a small arc it unlatches the train detent, the train runs and lifts the arm to its upper position and relocks the train. In this way the torque at the escape wheel and the impulse to the pendulum stay constant. The weighted arm continues to provide the same driving torque during the rewind. Other types of remontoire exist. Christiaan Huygens developed one using his endless-chain principle. In some, two local energy stores are provided so that one is rewinding while the other drives the escapement; they change from rewind to drive at regular intervals. Most turret clocks use weight-driven remontoires, although spring types are known. Taken to its logical conclusion, the remontoire can be fitted to provide a constant store of energy for impulsing the pendulum directly. This arrangement manifests itself in the gravity escapement. Some clocks have a remontoire and a gravity escapement, which is clearly absurd; one or the other is adequate.