Complex sentence: An inclinometer contains a ball sealed inside a curved glass tube, which also contains a liquid to act as a damping medium, in early aircraft the indicator was in effect a spirit level with the tube curved in the opposite direction and a bubble replacing the ball.
Split into simpler sentences: An inclinometer contains a ball sealed inside a curved glass tube, which also contains a liquid to act as a damping medium. The original form of the indicator is in effect a spirit level with the tube curved in the opposite direction and a bubble replacing the ball.