This famous esculent—the most agreeable and nutritious vegetable which decorates the Celtic table or the American board, which is to the Irish peasant what corn is to the Western emigrant—has been for some time subject to a dire disease, emphatically known as the Kot. Both here and in Europe the potato crop pays remarkably well, in good seasons here an acre of land producing 400 bushels of sound potatoes, which, at fifty cents a bushel, give $200 per acre as the result of a year's farming. When we consider the vast extent of land which is thus planted, we can easily conceive the enormous interest that depends upon a healthy and good crop, and as in the last few years the rot has been increasing in geographical extent, it becomes excessively important to discover its cause. In the year 1805 the Annual Register published an account of the disease, and attributed it to a small insect called the Aphis, and it stated that " hi some years the aphides are so numerous as to cause almost a total failure of the hop and potato plantations ; in other years the peas are equally injured, while exotics raised in stoves and greenhouses are frequently destroyed by the deprecations of these insects." From that time until 1846 this disease received but little attention; but when in that year a whole nation was pauperized by the failure of the potato crop, and thirty thousand fellow human beings died of actual starvation, it became necessary that men of science and friends of humanity should endeavor to discover the cause of this dire calamity, and to prevent its recurrence. These patient investigators set to work, and many avowed their belief that the cause was the growth of fungi in the cellular tissue of the tuber and vine, and many interesting pamphlets were published, and many eloquent lectures delivered, to prove this fact; but practical farmers remained sceptical, and as no remedies were proposed, they endeavored to conquer the disease by carefully selecting the seed, and paying more than usual attention to its cultivation. Still the' rot spread, and instead of being an accidental sickness, it has become a regular and periodic disease, wending its venomous way with as great certainty as cholera or small pox. In 1847 Mr. Alfred Smee, surgeon to the Bank of England, author of nn excellent work on electro-metallurgy, a patient observer, and an excellent microseop-ist, undertook the enormous difficulty of solving the great question—what the cause— where the cure—and in a little work, entitled " Smee on the Potato Plant," which he dedicates to His Koyal Highness Prince Albert, he gives the results of his investigation. A little insect, so small as to be scarcely visible to the naked eye, and which he calls the Aphis Vastalor, is in his opinion the culprit, and although he carefully explains all the symptoms of disease and all the features of the insect, yet, with the exception of recommending that the insect should he picked off some potatoes by way of trial, in his whole 150 pages he offers no remedy—suggests no cure. The Hon. Lyman Reed, of Maryland, has observed the larvae of what he calls the Aphis working at the seed tubers, roots and stalks, under ground, and consequently his observations seemingly tend t o confirm Mr. Smee, as the one saw the full-winged insect sucking the juices of the vines, and so destroying the plant, and depositing its eggs upon the leaf, and the young spreading rapidly, but all above ground and on the vine; Mr. Reed saw what he thought was the larvae of the same insect (Aphis Vastator) working on the tubers, &c, under ground. This is very good, nicely scientific, and pleasant to read about, hut no remedy has as yet been snggested, and for a very simple reason—the Aphis is not the culprit! Mr. Smee, in paragraph 245, says that " A very small insect, which is constantly to be seen upon tubers, is a small Acarus, which runs about the potato very nimbly. There is no reason whatever for believing that it causes the malady." Mr. Smee has been on the verge of making the discovery, but neglected to observe the Acanis, as .he calls it, closely, or he would have discovered what Mr. Alexander Henderson, of Buffalo, N. Y., has discovered, namely, that the Phylocoris Linealaris, of P. de Barrce, the Capris Oblini-atus of Say, or the Acarus of Smee, is the cause of the potato rot. His reasons for coming to this conclusion, as the result of much investigation and observation, we will now give, as we have had the benefit -of his personal account, and from his plain, straightforward tale, there would seem to be no doubt that he is the discoverer of the cause and cure of the potato rot. There may be many rots, we do not know; investigation can only prove that; but that Mr. Henderson can nure one rot, similar to the one which was in Great Britain in 1847, there is little doubt, and it only remainfor extended experiments," observers in different parts of the world, to prove whether his sim- ple remedy is applicable alike to all rots everywhere. One thing appears certain: Mr. Hendersoa has discovered that the Phylocoris are the primary cause of the potato rot, strictly so called, which first appeared in 1845, and which is identical with that of the present season. If a tuber be examined with a microscope just before planting, on it may be seen a small, yellowish, translucent oval object, secured, as is common with insects' eggs, by a gummy substance to the potato. This will produce unsound potatoes, and the egg is that of the Phylocoris. When the toher is planted at the ordinary depth, this egg hatches, but if the potato is planted deep, the egg is killed, and therefore deep planting is one remedy, because air and light are prevented from coming to the delicate egg. After a sufficient amount of warmth and moisture has been obtained by the egg, the shortest time that haa yet been observed being six days, the shell opens along its greater axis, and out comes the small insect, without wings, from about the twentieth to a twelfth of an inch long. It has six perfect legs, two attennse, a proboscis, and a pair of brilliant black eyes. The proboscis is about two-thirds of its body in length, and one-third of its length from the head is thick, seen coiled upon itself at c, Fig. 3 (which is an enlarged view of the perfect insect, taken, with the other views, from actual plants, tubers and insects, brought to us by Mr. Henderson), and the remainder is flexible and needle-like. It contains three tubes, through one of which it sucks up the juice of the plant for its nutriment ; through another it probably ejects a poison into the plant, and through the other it may perform part of its respiration. The young insect being born alive, instantly requires nutriment, and commenees feeding upon the seed, which, without the young are very numerous, does not perceptibly interfere with its growth. According to the amount of heat and moisture in the soil, this goes on from two and a half to three months, when the insect gets wings, and the vine has attained its full growth. The insect has all this time been working at the tuber (Fig. 2), absorbing much of its nutritive juice, and injecting a poison, which at first appears in spots, as seen at a, Fig. 4. These rapidly spread to blotches, b, daily becoming more rotten, as c, and at last leaving very little of the sound potato, d. The winged insect, tired of his dark underground quarters, moves a few stories' higher, and settles himself upon the leaves of the vine, as seen at 1. They naturally attack the leaves and main stem, which, having their juices taken from them, wither and die, leaving little save their silicious and carbonaceous skeletons, and producing the appearance of the rot. The best evidence that a poison Is also injected into the plant, is found in the fact that fungi in great abundance make their appearance, and these, as is well known, are generally the result of putrefactive fermentation. Sometimes, in cold and stormy weather, the insect again descends and feeds upon the potato, which by this time is covered with fungi also. In the winter they emigrate, and nestling among the warm leaves of the mullen plant, endearor to keep alive until the succeeding spring. Mr. Henderson has been engaged observing these insects since 1850, although for five years previously he had investigated the causes of the potato rot. On page 382 of the present volume of the SCIENTIFIC AMERICAN, we gave a brief outline of Mr. H.'e discoveries, from the Buffalo Commercial, which was the first newspaper that gave an. extended notice of these facts. Feeling the importance of the subject, we have given this much space to it, knowing that the majority of our readers will look with eagerness for Mr. H.'s simple remedies, which are, killing the egg by sprinkling quick-lime upon the seeds—preventing its development by deep planting, by hoeing up well round the vines, and filling up the cracks in the soil by pressure—or by preserving an old Scotch method of planting, which is as follows : The ground is plowedabout a foot deep, the manure put in, with three to four inches of soil on that, and then the potato planted. Crops set in this way have never failed, the vines sometimes being attacked, but the tubers always remaining sound. We hope that many of our readers will set to work and experiment on this matter, and although the bug is very active and lively, he may be caught by shaking the vine quickly and picking him up. Mr. Henderson intends going to Europe, to bring his discoveries before the agriculturists of England and France, from whom we sincerely hope that he will meet with that success and attentive consideration that his patient investigation so richly deserve.8. SCANDINAVIA.—Professor Paul C. Sinding, late of Copenhagen, Denmark, now Professor of Scandinavian Languages and Literature in the University of New York, is about to issue a History of Scandinavia in one volume. We hail this volume from this distinguished source with much pleasure, for how little indeed is known by our most careful students of history concerning this people! The little we have read of their manners and customs, moral, social, political, religious and industrial only excites our appetite to know more about them. ATLANTIC TELEGRAPH AND FRESH NEWS. —A scientific gentleman put the enquiry in our office the other day, it" we supposed the news communicated through the Atlantic telegraph, 2,000 miles under the salt water, could be fresh when it reached the shore.