354 LECTURE XXX. 



arranged. Pipes of lead were, however, less frequent than at present, from 

 an apprehension of the poisonous quality of the metal, which was not wholly 

 without foundation. Some say that the ancients had no chimnies, but 

 whatever may be the authorities, the opinion is extremely improbable. 



It was in the middle ages that navigable canals began to be considerably 

 multiplied, first in China, and afterwards in other parts of the world. The 

 canal from the Trent to the Witham, which is the oldest in England, is said 

 to have been dug in 1134. The date of the earliest windmills has been re- 

 ferred to the year 1299. The invention of gunpowder possesses perhaps an 

 equal claim with theartof printing, to the honour of being considered as consti- 

 tuting the most marked feature, that distinguishes the character of ancient from 

 that of modern times; its introduction must necessarily have tended to 

 produce material alterations, and perhaps improvements, in the habits of 

 nations and of individuals. It is said to have been known long since to the 

 Chinese, and our countryman Roger Bacon was evidently acquainted with 

 its properties; but it was not actually employed in Europe or in its neighbour, 

 hood till about the year 1 330 ; and the earliest artillery appears to have been 

 that which was used by the Moors, at the siege of Algesiras, in 1334. King 

 Edward had four pieces of cannon at the memorable battle of Cressy, in 

 1346. 



About the year 1600, Galileo made the important discovery of the eflPects of 

 the weight and pressure of the atmosphere, in the operation of suction, and in 

 various other phenomena. Before his time, it was generally supposed that 

 water was raised by a sucking pump, on account of the impossibility of the 

 existence of a vacuum : if, however, a vacuum had been impossible in'na- 

 ture,the water would have followed the piston to all heights.however great, but 

 Galileo found that the height of its ascent was limited to about 34 feet, and con- 

 cluded that the weight of a column of this height was the measure of the magni- 

 tude of the atmospherical pressure. His pupil Torricelli afterwards confirmed 

 the explanation, by showing that a column of mercury was only supported 

 when its weight was equal to that of a column of -water standing on the 

 same base; hence the vacuum obtained by means of mercury is often called 

 the Torricellian vacuum. Torricelli corrected also, in 1644, the mistake of 

 Castelli respecting the quantities of water discharged by equal orifices, at 



