instruction:
Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.
question:
Fact 1: a barometer is used to measure air pressure. 
Fact 2: Uses Elemental mercury is used in thermometers, barometers, and pressure-sensing devices.
answer:
Elemental mercury helps measure air pressure.


question:
Fact 1: when a river floods , that river deposits soil in flood plains. 
Fact 2: Soil erosion harms farm and grazing lands and increases flooding and desertification.
answer:
When a river floods, that river harms farm and grazing lands.


question:
Fact 1: Some viruses live in a dormant state inside the body. 
Fact 2: When ferns are dormant it is as if they are hibernating.
answer:

Some viruses live in a hibernating state inside the body.