Speculations about the weather occupy much space in the writings of the Greek philosophers, and a formal treatise on meteorology, written by Aristotle (fourth century B. C.), remained the standard work on this subject for two thousand years. More or less systematic weather records were kept by the Greeks long before the Christian era, and they produced a number of almanacs, in the shape of marble tablets, showing the average winds and weather for particular dates throughout the year. A copious collection of the weather indications found in both Greek and Roman almanacs, dating back to the fifth century B. C., has been made by Dr. Gustav Hellmann. Some of the meteorological instruments used today have a very respectable antiquity. Ancient statistics of the rainfall of India, recently brought to light, show that some sort of rain gauge must have been in use in that country in the fourth century before our era. Measurements of rainfall were made in Palestine in the first century A. D. The only other meteorological instrument dating back to classical antiquity, so far as known, is the weather vane. The Tower of the Winds, at Athens, built about a century before the Christian era, originally bore at its summit a vane in the shape of a bronze Triton, holding in his hand a wand, which was designed to point at one or another of the eight symbolical figures of the principal winds surrounding the octagonal tower, thus showing which way the wind was blowing at the time. The Roman writer Varro has left us a description of a vane that could be read indoors by means of a dial on the ceiling.