Specific gravity is the ratio of density of a substance compared to the density of fresh water at 4°C (39°F)
参考译文:比重是某物质的密度与水的密度之比。
解答: compared to the density of fresh water at 4°C (39°F)为过去分词作定语。相当于是which is compared to the density of fresh water at 4°C (39°F)