A basic church can be constructed using mud brick, wattle and daub, split logs, or rubble. Its roof might be made of thatch, shingles, corrugated iron, or banana leaves. However, beginning in the fourth century, church groups attempted to build church buildings that were both lasting and artistically attractive. These early churches used concrete for their foundations and walls. They often had flat roofs covered with tiles or slates. Concrete was first developed in China about 2,000 years ago. It was introduced into Europe around 500 AD by Arab merchants. Concrete has been used for large structures such as bridges, but it was not suitable for use in building small houses. In the 17th century, French scientist Blaise Pascal invented the pneumatic tube system, which is now used in subway stations all over the world. Pascal also invented a machine called a "pneumatograph," which was the first air-conditioned room. It functioned by passing air through tubes wrapped around a cylinder attached to a fan; the wind from the fan moved through the tubes, cooling off the interior space. This invention helped make big buildings possible again after the great fire of 1668 destroyed much of London. In the United States, some churches are built with wood, while others are built with concrete. The Church of Jesus Christ of Latter-day Saints (LDS Church) uses steel instead.