Makhshirin, Chapter Five, Mishnah Four
Our mishnah discusses one who uses a stick to measure a cistern. Does the water that comes up with the stick cause produce to be susceptible to impurity?
1) One who measures a cistern whether for its depth of for its breadth, it comes under the law of if water be put’ the words of Rabbi Tarfon.
2) But Rabbi Akiva says: if [it was measured] for its depth, it comes under the law of if water be put; but if for its breadth, it does not come under the law of if water be put.
Section one: Rabbi Tarfon says that since the stick will likely get wet, even if he is only measuring the breadth, in essence he wanted it to get wet. Without getting it wet, he would not have been able to measure the cistern. Therefore, no matter whether he is measuring the length or the depth, the water causes susceptibility.
Section two: Rabbi Akiva says that only if he is measuring for depth does the water cause susceptibility for in this case he needs to see the mark that the water makes on the stick, i.e. how high it goes. When measuring for breadth, even though it gets wet, he doesn’t need to see the mark and therefore it doesn’t cause susceptibility.