228 M. Arago ow the Thermometrical State 



we cannot safely draw any deduction from them. Take, for 

 example, the vine. A passage from Herodotus will inform us, 

 that the Egyptians did not cultivate it at all ; while, at the 

 same time, we shall find Athanaeus boasting of the wines of 

 Alexandria. Should we be solicitous of discovering the south- 

 ern limits of its cultivation, we shall find in Theophrastus, ex- 

 press mention of vines growing in the immediate neighbourhood 

 of Elephantina. But such information will always be useless, 

 for the question about climate is to be elucidated, not by state- 

 ments of the latitudes in which the vine grows, but by those in 

 which it is incapable of supplying good wine. The documents 

 concerning the palm-tree are not more satisfactory. According 

 to Strabo, these trees were barren, or at least did not furnish 

 good dates, at Alexandria and throughout the delta. And then 

 again it is stated that the whole of the Lower Egypt was cover- 

 ed with them. It is wise, then, to leave such obscure passages, 

 which often rested only on doubtful reports. Let us now pro- 

 ceed to the study of our own climes. 



And here let it be again remarked, that we are about to 

 occupy our attention with changes distinctly local, without pre- 

 tending to extend even throughout a whole kingdom, that 

 which may have happened at a particular part of it. Every 

 other method of examination will assuredly fail of that preci- 

 sion, which, in the present time, we have a right to expect in a 

 scientific discussion. Should it be wished, that we should also 

 for a moment follow Daines Barrington, the Abbe Mann, and 

 many other naturalists, in the efforts they have made to prove 

 that the climate of the whole continent of Europe, and of some 

 parts of Asia, has been considerably deteriorated in the course 

 of ages ? Should it be desired, that, after the example of these 

 authors, we should proceed by taking the exceptions, and the 

 extraordinary phenomena only ? If so, analogous phenomena 

 quite as extraordinary, and quite recent, present themselves in 

 abundance. 



We are told to read Diodorus Siculus, and thence learn that, 

 in ancient times, the rivers throughout France were often frozen 

 in winter, so that infantry and even cavalry, chariots, and the 

 weightiest equipages crossed them without danger. So, too, ac- 

 cording to Dion Cassius, Trajan's famous bridge across the 



