Complex sentence: Before the mid- 19th century these shells were usually made of solid materials and relied on kinetic energy to have an effect, but since that time they are more often filled with high-explosives (see artillery).
Split into simpler sentences: Before the mid- 19th century, these shells were usually made of solid materials and relied on kinetic energy to have an effect. However, since that time they are more often filled with high-explosives (see artillery).