A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
num 10:2THE USE OF THE SILVER TRUMPETS. (Num. 10:1-36)
Make thee two trumpets of silver--These trumpets were of a long form, in opposition to that of the Egyptian trumpets, with which the people were convened to the worship of Osiris and which were curved like rams' horns. Those which Moses made, as described by JOSEPHUS and represented on the arch of Titus, were straight, a cubit or more in length, the tubes of the thickness of a flute. Both extremities bore a close resemblance to those in use among us. They were of solid silver--so as, from the purity of the metal, to give a shrill, distinct sound; and there were two of them, probably because there were only two sons of Aaron; but at a later period the number was greatly increased (Jos 6:8; Ch2 5:12). And although the camp comprehended 2,500,000 of people, two trumpets would be quite sufficient, for sound is conveyed easily through the pure atmosphere and reverberated strongly among the valleys of the Sinaitic hills.
num 10:3when they shall blow with them--There seem to have been signals made by a difference in the loudness and variety in the notes, suited for different occasions, and which the Israelites learned to distinguish. A simple uniform sound by both trumpets summoned a general assembly of the people; the blast of a single trumpet convoked the princes to consult on public affairs; notes of some other kind were made to sound an alarm, whether for journeying or for war. One alarm was the recognized signal for the eastern division of the camp (the tribes of Judah, Issachar, and Zebulun) to march; two alarms gave the signal for the southern to move; and, though it is not in our present Hebrew text, the Septuagint has, that on three alarms being sounded, those on the west; while on four blasts, those on the north decamped. Thus the greatest order and discipline were established in the Israelitish camp--no military march could be better regulated.
num 10:8the sons of Aaron the priests shall blow with the trumpets, &c.--Neither the Levites nor any in the common ranks of the people could be employed in this office of signal giving. In order to attract greater attention and more faithful observance, it was reserved to the priests alone, as the Lord's ministers; and as anciently in Persia and other Eastern countries the alarm trumpets were sounded from the tent of the sovereign, so were they blown from the tabernacle, the visible residence of Israel's King.
num 10:9If ye go to war--In the land of Canaan, either when attacked by foreign invaders or when they went to take possession according to the divine promise, "ye [that is, the priests] shall blow an alarm." This advice was accordingly acted upon (Num 31:6; Ch2 13:12); and in the circumstances it was an act of devout confidence in God. A solemn and religious act on the eve of a battle has often animated the hearts of those who felt they were engaged in a good and just cause; and so the blowing of the trumpet, being an ordinance of God, produced that effect on the minds of the Israelites. But more is meant by the words--namely, that God would, as it were, be aroused by the trumpet to bless with His presence and aid.
num 10:10Also in the day of your gladness, and in your solemn days--Festive and thanksgiving occasions were to be ushered in with the trumpets, as all feasts afterwards were (Psa 81:3; Ch2 29:27) to intimate the joyous and delighted feelings with which they engaged in the service of God.
num 10:11It came to pass on the twentieth day of the second month, in the second year, &c.--The Israelites had lain encamped in Wady-Er-Rahah and the neighboring valleys of the Sinaitic range for the space of eleven months and twenty-nine days. (Compare Exo 19:1). Besides the religious purposes of the highest importance to which their long sojourn at Sinai was subservient, the Israelites, after the hardships and oppression of the Egyptian servitude, required an interval of repose and refreshment. They were neither physically nor morally in a condition to enter the lists with the warlike people they had to encounter before obtaining possession of Canaan. But the wondrous transactions at Sinai--the arm of Jehovah so visibly displayed in their favor--the covenant entered into, and the special blessings guaranteed, beginning a course of moral and religious education which moulded the character of this people--made them acquainted with their high destiny and inspired them with those noble principles of divine truth and righteousness which alone make a great nation.
num 10:12wilderness of Paran--It stretched from the base of the Sinaitic group, or from Et-Tyh, over that extensive plateau to the southwestern borders of Palestine.
num 10:13the children of Israel took their journey . . . by the hand of Moses--It is probable that Moses, on the breaking up of the encampment, stationed himself on some eminence to see the ranks defile in order through the embouchure of the mountains. The marching order is described (Num. 2:1-34); but, as the vast horde is represented here in actual migration, let us notice the extraordinary care that was taken for ensuring the safe conveyance of the holy things. In the rear of Judah, which, with the tribes of Issachar and Zebulun, led the van, followed the Gershonites and Merarites with the heavy and coarser materials of the tabernacle. Next in order were set in motion the flank divisions of Reuben and Ephraim. Then came the Kohathites, who occupied the center of the moving mass, bearing the sacred utensils on their shoulder. They were so far behind the other portions of the Levitical body that these would have time at the new encampment to rear the framework of the tabernacle before the Kohathites arrived. Last of all, Dan, with the associated tribes, brought up the rear of the immense caravan. Each tribe was marshalled under its prince or chief and in all their movements rallied around its own standard.
num 10:29Hobab, the son of Raguel the Midianite--called also Reuel (the same as Jethro [Exo 2:18, Margin]). Hobab, the son of this Midianite chief and brother-in-law to Moses, seems to have sojourned among the Israelites during the whole period of their encampment at Sinai and now on their removal proposed returning to his own abode. Moses urged him to remain, both for his own benefit from a religious point of view, and for the useful services his nomad habits could enable him to render.
num 10:31Leave us not, I pray thee . . . and thou mayest be to us instead of eyes--The earnest importunity of Moses to secure the attendance of this man, when he enjoyed the benefit of the directing cloud, has surprised many. But it should be recollected that the guidance of the cloud, though it showed the general route to be taken through the trackless desert, would not be so special and minute as to point out the places where pasture, shade, and water were to be obtained and which were often hid in obscure spots by the shifting sands. Besides, several detachments were sent off from the main body; the services of Hobab, not as a single Arab, but as a prince of a powerful clan, would have been exceedingly useful.
num 10:32if thou go with us . . . what goodness the Lord will show unto us, the same will we do unto thee--A strong inducement is here held out; but it seems not to have changed the young man's purpose, for he departed and settled in his own district. (See on Jdg 1:16 and Sa1 15:6).
num 10:33they departed . . . three days' journey--the first day's progress being very small, about eighteen or twenty miles.
ark of the covenant of the Lord went before them--It was carried in the center, and hence some eminent commentators think the passage should be rendered, "the ark went in their presence," the cloud above upon it being conspicuous in their eyes. But it is probable that the cloudy pillar, which, while stationary, rested upon the ark, preceded them in the march--as, when in motion at one time (Exo 14:19) it is expressly said to have shifted its place.
num 10:35when the ark set forward that Moses said, Rise up, Lord, and let thine enemies be scattered--Moses, as the organ of the people, uttered an appropriate prayer both at the commencement and the end of each journey. Thus all the journeys were sanctified by devotion; and so should our prayer be, "If thy presence go not with us, carry us not hence" [Exo 33:15].