From the experiment described in Wheatstone’s paper (_Philosophical Transactions_ for 1834), it would appear that the human eye is capable of perceiving phenomena of light whose duration is limited to the millionth part of a second. In Professor Airy’s experiments with the electric telegraph to determine the difference of longitude between Greenwich and Brussels, the time spent by the electric current in passing from one observatory to the other (270 miles) was found to be 0·109″ or rather more than _the ninth part of a second_; and this determination rests on 2616 observations: a speed which would “girdle the globe” in ten seconds. IDENTITY OF ELECTRIC AND MAGNETIC ATTRACTION. This vague presentiment of the ancients has been verified in our own times. “When electrum (amber),” says Pliny, “is animated by friction and heat, it will attract bark and dry leaves precisely as the loadstone attracts iron.” The same words may be found in the literature of an Asiatic nation, and occur in a eulogium on the loadstone by the Chinese physicist Knopho, in the fourth century: “The magnet attracts iron as amber does the smallest grain of mustard-seed. It is like a breath of wind, which mysteriously penetrates through both, and communicates itself with the rapidity of an arrow.” Humboldt observed with astonishment on the woody banks of the Orinoco, in the sports of the natives, that the excitement of electricity by friction was known to these savage races. Children may be seen to rub the dry, flat, and shining seeds or husks of a trailing plant until they are able to attract threads of cotton and pieces of bamboo-cane. What a chasm divides the electric pastime of these naked copper-coloured Indians from the discovery of a metallic conductor discharging its electric shocks, or a pile formed of many chemically-decomposing substances, or a light-engendering magnetic apparatus! In such a chasm lie buried thousands of years, that compose the history of the intellectual development of mankind.--_Humboldt’s Cosmos_, vol. i. THEORY OF THE ELECTRO-MAGNETIC ENGINE.