Answer:
a device for collecting and measuring rain as it falls
You can use refraction which is the bending of light through a transparent material. Also the freezing and boiling points of water. One last one is the Cartesian five experiment, this shows the density of water. Hope this helped.
In the population with a large number of individuals, the mutation would only have a small impact, while a population with only 18 individuals would result in a very large i off as the mutation would be in a greater percentage of the population.