eighteen cubits:

That is, nearly thirty feet, English measure. But in the parallel place in Chronicles, these pillars are said to thirty-five cubits high. Tremellius reconciles this difference by observing, that the common cubit was but one-half of the cubit of the sanctuary; so that eighteen of the one would make thirty-six of the other; from which, if we deduct one cubit for the base, there will remain thirty-five. Notwithstanding the names of these pillars, they seem to have supported no part of the building, and appear to have been formed for ornament; and were no doubt also emblematical. The right pillar was called Jachin, which signifies, "He will establish;" while that on the left was named Boaz, "In it is strength." Some think they were intended for memorials of the pillars and cloud of fire, which led Israel through the wilderness; but Henry supposes them designed for memorandums to the priests and others that came to worship at God's door.

1. To depend upon God only, and not upon any sufficiency of their own, for strength and establishment in all their religious exercises.
2. It was a memorandum to them of the strength and establishment of the temple of God among them.
When the temple was destroyed, particular notice is taken of the breaking up and carrying away of these brazen pillars, "And the pillarsª of brassª that¹ [were] in the houseª of the LORD,ª and the bases,ª and the brasenª seaª that¹ [was] in the houseª of the LORD,ª did the Chaldeesª break in pieces,ª° and carriedª°¹ the brassª of them to Babylon.ª" {2Kg 25:13} + "The heightª of the oneª pillarª [was] eighteenªª cubits,ª and the chapiterª upon¹ it [was] brass:ª and the heightª of the chapiterª threeª cubits;ª and the wreathen work,ª and pomegranatesª upon¹ the chapiterª round about,ª all¹ of brass:ª and like unto these¹ had the secondª pillarª with¹ wreathen work.ª" {2Kg 25:17}, which had been the tokens of its establishment, and would have been still so, if they had not forsaken God.