Final answer:
Details that indicate an increase in mosquito numbers are the rising counts from 20-40 to 100-125 on the hand in mere seconds and the depiction of the Barren Grounds as a heavily infested area.
Step-by-step explanation:
The question asks which two details in the passage suggest that the number of mosquitoes were increasing. The first detail from the given passage is the quantitative increase in mosquito counts from 20 to 40 in early July to 100 to 125 mosquitoes on the hand in just 5 seconds in the Barren Grounds. The second detail is the description of the Barren Grounds as a land of open breezy plains and cold water lakes, which articulates an apparent escalation in the mosquito infestation in this region, making it a 'kingdom of such a scourge'.