The exact duration of a flash of lightning is a question on which no settled opinion has yet been accepted generally by scientific men. Indeed, the most widely different statements have been made on the subject, quite recently, by the highest authorities, each speaking apparently with unhesitating confidence. Thus, for example, Professor Mascart describes an experiment, which he says was made by Wheatstone, and which showed that a flash of lightning lasts for less than _one_-thousandth of a second;[2] Professor Everett describes the same experiment, without saying by whom it was made, and gives, as the result, that “the duration of the illumination produced by lightning is certainly less than the _ten_-thousandth of a second;”[3] Professor Tyndall, in his own picturesque way, tells us that “a flash of lightning cleaves a cloud, appearing and disappearing in less than the _hundred_-thousandth of a second;”[4] and according to Professor Tait, of Edinburgh, “Wheatstone has shown that lightning certainly lasts less than the _millionth_ of a second.”[5] =Experiments of Professor Rood.=--I cannot say which of these statements is best supported by actual observation; for none of the writers I have quoted gives any reference to the original memoir from which his statement is derived. As far as my own reading goes, I have only come across one original record of experiments, made directly on the flash of lightning itself, with a view to determine the period of its duration. These experiments were carried out by Professor Ogden Rood, of Columbia College, New York, between the years 1870 and 1873, and are recorded in the _American Journal of Science and Arts_.[6] For the description of his apparatus, and for the details of his observations, I must refer you to the memoir itself; but I may tell you briefly that the results at which he arrived, if they be accepted, must lead to a considerable modification of the views previously entertained on the subject. In the first place, he satisfied himself that what appears to the eye a single flash of lightning is usually, if not always, multiple in its character; consisting, in fact, of a succession of distinct flashes, which follow one another with such rapidity as to make a continuous impression on the retina. Next, he proceeded to measure approximately the duration of these several component flashes; and he found that it varied over a wide range, amounting sometimes to fully the twentieth of a second, and being sometimes less than the sixteen-hundredth of a second.