Page:Popular Science Monthly Volume 14.djvu/539

Rh three cases caused by drinking-water a period of incubation of twenty-one days. In our own epidemic one case occurred in fourteen days after drinking the water, the average time being about twenty days. The general experience at the present time points to an incubation of about three weeks; but it may vary from two to four weeks, or even longer.

It is an interesting question, but one to which I cannot give a satisfactory answer, Why do not all who are exposed to the infecting cause suffer from the disease? In some cases it may be due to idiosyncrasy. Some people resist powerfully the encroachment of all infectious diseases, while others seem fated to have an opportunity of testing in their own person every prevailing malady. Experience teaches, however, that typhoid fever, unlike many other diseases of the group, is favored in its development by unhygienic surroundings. Bad air, bad food, uncleanliness, and over-population of a house or quarter of a city, create a condition in the system favorable to a rapid and virulent development of the fever after an infection by the germs. We know nevertheless, that no house nor person is exempt from the disease after receiving the exciting cause. Prince and peasant alike have to bow to the malignant potency of these infecting germs.



LISHA GRAY, the inventor of the Speaking Telephone, was born at Barnesville, Belmont County, Ohio, August 2, 1835. During his boyhood he was profoundly interested in all the phenomena of nature, and had an intense desire, whenever he saw any manifestation of physical force, to become acquainted with the secret of its operation. Among all the phenomena throughout the domain of physics, nothing took such hold upon his mind as that exhibited in the various effects produced by the action of electricity, and he read whatever he could find relating to this subject with the same eagerness and interest that most boys would read "Robinson Crusoe" or the "Arabian Nights."

While yet a boy he constructed a Morse register, all the parts of which were made of wood, with the exception of the magnet, armature, and embossing point in the end of the lever (which latter he made by filing a nail down to a point). He had the magnet bent into a U-form by a blacksmith, and then wound it with brass bell-wire, which was insulated with strips of cotton cloth wrapped around it by hand. For a battery he made use of a candy-jar, in which he placed coils of sheet copper and zinc, with a solution of blue vitriol. With these materials he succeeded in making a very good electro-magnet, which would 