Exodus 27:1

Altar, of holocausts, in the open air, before the tabernacle. (Tirinus) --- Four square, or five cubits in length and breadth, and three in height, which the Rabbins measure from the grate, (ver. 5,) or middle of the altar's height. So high the altar was sunk in the earth, (Calmet) or was built of unhewn stone, on which the wood of the altar rested, being secured by plates of brass above, from the heat of the fire. It was hollow within, and had neither top nor bottom fixed to it. (Menochius)