The first altar mentioned in the Word was the one built by Noah after he came out of the ark, after being saved from the great flood. On that altar, he sacrificed clean animals to the Lord.

Mountains represent the Lord because of their height; we need to raise our thoughts above worldly things when "talking" with the Lord. An altar is a small artificial mountain. When it's used in worship, it can call to mind this raising of thought. The fire and smoke that rise from an altar are symbolically being sent to the Lord.

Most altars were made from unhewn stones. Stones represent truths. Unhewn stones - ones that have not been shaped by men - represent truths from the Word, truths that have not been adulterated.

The clean beasts to be sacrificed represent good things, charitable acts done because they are right. The clean birds represent thoughts about doctrine and actions, and about what is right. Presenting these things is an acknowledgment that we have them from the Lord, and a giving thanks to Him for them.

In the Israelitish Tabernacle, the altar of burnt offering represented the acknowledgment of good and the altar of incense that of truth. For this reason this larger altar, which was outside by the door, was made of brass which signifies natural good, while the altar of incense was made of gold, which signifies love to the Lord from whom comes truth.

8106. 'During the daytime in a pillar of cloud' means that when there was a state of enlightenment, this was moderated by a dimming of truth. This is clear from the meaning of 'during the daytime' or 'in the day' as in a state of enlightenment, for the times of day - morning, midday, evening, and night - correspond to degrees of enlightenment in the next life, that is, of intelligence and wisdom, 5672, 5962, 6110, so that 'the day' is a state of enlightenment or clear perception, and 'the night' a state of unenlightenment or dimmed perception, 7680; and from the meaning of 'cloud' as a dimming of truth, since a cloud takes away the brilliance of the light from the sun and also moderates it.

[2] Various places in the Word say that Jehovah appeared in a cloud, that He was clothed with a cloud, or that 'clouds were under His feet'. In those places 'cloud' is used to mean a dimming of truth; in particular the literal sense of the Word is meant, for in comparison with the internal sense the literal sense is a dimmed presentation of truth, see the Preface to Genesis 18, and also 4391, 5922, 6343, 6752. This was meant by 'the cloud' when the Lord appeared in glory to Peter, James, and John, Luke 9:34; when He appeared to the people from Mount Sinai, and to Moses when he went in to Him there, Exodus 19:9; 20:21; 24:15-18; 34:5. The same was also meant by what the Lord declared so many times, that He would come in the clouds of heaven, Matthew 24:30; 26:63-64; Mark 13:26; 14:61-62; Luke 21:27.

[3] The literal sense of the Word is called 'a cloud' because the internal sense, which is called 'the glory', cannot be understood by anyone unless he has been regenerated and therefore enlightened. If the internal sense of the Word, or God's truth in its glory, were to appear in front of someone who has not been regenerated it would be like thick darkness in which he would see absolutely nothing, and also would leave him blind, that is, with no belief at all.

From all this one may see what the description 'a cloud during the daytime' means, namely a dimming of truth and, when it has reference to the Word, the literal sense.

[4] The expressions 'in a pillar of cloud' and 'in a pillar of fire' are used because 'a pillar' means a load-bearing support, as in Jeremiah 1:18; Psalms 75:3; Revelation 3:12; Job 9:6; and it is used to refer to the natural level, since the natural level is a kind of support or pedestal for the spiritual level. For the spiritual level terminates at the natural level and rests on it. This explains why the feet of an angel coming down from heaven looked like pillars of fire, Revelation 10:1; for the natural level is meant by 'feet', 2162, 3147, 3761, 3986, 4280, 4938-4952, 5327, 5328.