Genesis 21:15

And she cast the child - הילד את ותשלך vattashlech eth haiyeled , and she sent the lad under one of the shrubs, viz., to screen him from the intensity of the heat. Here Ishmael appears to be utterly helpless, and this circumstance seems farther to confirm the opinion that he was now in a state of infancy; but the preceding observations do this supposition entirely away, and his present helplessness will be easily accounted for on this ground:

1. Young persons can bear much less fatigue than those who are arrived at mature age.

They require much more fluid from the greater quantum of heat in their bodies, strongly marked by the impetuosity of the blood; because from them a much larger quantity of the fluids is thrown off by sweat and insensible perspiration, than from grown up or aged persons.

3. Their digestion is much more rapid, and hence they cannot bear hunger and thirst as well as the others. On these grounds Ishmael must be much more exhausted with fatigue than his mother.