Page:Popular Science Monthly Volume 41.djvu/880

860 and accordeons is a specialty of Gera. The manufacture of physical, optical, and medical instruments is one of the special occupations in the forest of Thuringia. For many years the people of Ilmenau, Mauebach, and Stutzerbach have devoted themselves almost entirely to the construction of thermometers, barometers, baroscopes, and hygrometers. Whole families are engaged in the work; and children are set upon it from a very early age. It is a surprising sight, on roads distant from the centers of trade, to see whole trains of wagons loaded with physical instruments. The products of this manufacture are much appreciated in Germany. Their construction is perfect; their accuracy is guaranteed by a royal commission at Ilmenau; and many of the universities and doctors supply themselves directly from the country.

Snails of Mountain and Plain.—The influence of the medium on variation has been specially studied by M. A. Locard in the case of land-mollusks, or snails. First among these elements considered is altitude. It seems a simple matter, but is really complicated, and includes among other elements those of temperature, light, ventilation, and food resources, the respective actions of which are hard to separate. The author takes them all in one. The same species of snails are often found at the level of the sea and in the mountains. But the number of individuals greatly diminishes as the altitude increases, under the influence, apparently, of the variations in certain vital conditions. There are species, however, so well adapted to life in elevated situations that they do not thrive elsewhere. The Helix alpina, for instance, lives only in the mountains. Though individuals of the species readily and constantly stray into the valleys, they do not form stocks there. As between the species of the mountains and of the plains, the former are smaller, and have thinner and plainer colored shells. With increase of heat, below the degree of intensity at which existence is threatened, the size increases. Many species attain double the size they reach in France, while species transported from Algeria to France shrink to one fourth the dimensions they have in their native habitat. The character of the shell is affected by variations in the soil. Calcareous districts are rich in mollusks, while those in which the soil is silicious are poor in them, and the animals themselves are smaller and less vigorous. The difference may be experimentally verified by feeding half a lot of snails upon plants growing in sandy ground and the other half with plants from calcareous soil; a great difference will be perceived in the size of the shells.

Size and Shape of Rain-drops.—Mr. E. J. Lowe has made more than three hundred sketches of rain-drops, and has gathered some interesting facts respecting their variation in size, form, and distribution. Sheets of slate in a book form, which could be instantly closed, were employed. These were ruled in inch squares, and after exposure the drops were copied on sheets of paper ruled like the slates. Some drops produce a wet circular spot; while others, falling with greater force, have splashes around the spots. The same drop varies considerably in the amount of water it contains. The size of the drop ranges from an almost invisible point to at least two inches in diameter. Occasionally large drops fall that must be more or less hollow, as they fail to wet the whole surface inclosed within the drop. Besides the ordinary rain-drops the author exhibited to the Royal Meteorological Society diagrams showing the drops produced by a mist floating along the ground, and also the manner in which snow-flakes, on melting, wet the slates.

Geographical Work of 1891.—In his annual address as President of the Royal Geographical Society Sir Montstuart E. Grant Duff, reviewing the incidents of the geographical exploration accomplished during the year, noticed Mr. Merzbacher's work in the Caucasus and Mr. Howell's ascent of Oraefa Jokull in Iceland as the chief mountaineering feats. In Asia military exploration had gone on steadily on the northern frontiers, and the society was making efforts to have the results of such work made more accessible to the public. Lord Lamington's journey in the Shan states, and Captain Bower's and Dr. Thorold's adventurous crossing of Thibet also opened up new ground. In Africa, Mr. A. E. Floyer crossed the Egyptian Desert from Assouan to the Red Sea; and in the region of the Great Lakes