86o 



THE POPULAR SCIENCE MONTHLY. 



and accordeons is a specialty of Gcra. The 

 manufacture of physical, optical, and medi- 

 cal instruments is one of the special occupa- 

 tions 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 dis- 

 tant from the centers of trade, to see whole 

 trains of wagons loaded with physical instru- 

 ments. The products of this manufacture 

 are much appreciated in Germany. Their 

 construction is perfect; their accuracy is 

 guaranteed by a royal commission at Ilme- 

 nau ; and many of the universities and doctors 

 supply themselves directly from the country. 



Snails of Mountain and Plain. The in- 

 fluence 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 compli- 

 cated, 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 varia- 

 tions 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 indi- 

 viduals of the species readily and constantly 

 stray into the valleys, they do not form stocks 

 there. As between the species of the mount- 

 ains 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 mol- 



lusks, while those in which the soil is sili- 

 cious are poor in them, and the animals them- 

 selves are smaller and less vigorous. The 

 difference may be experimentally verified by 

 feeding half a lot of snails upon plants grow- 

 ing 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 varia- 

 tion in size, form, and distribution. Sheets 

 of slate in a book form, which could be in- 

 stantly 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 in- 

 visible point to at least two inches in di- 

 ameter. 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 pro- 

 duced 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 geo- 

 graphical 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 mount- 

 aineering feats. In Asia military explora- 

 tion 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 cross- 

 ing of Thibet also opened up new ground. In 

 Africa, Mr. A. E. Floyer crossed the Egyp- 

 tian Desert from Assouan to the Red Sea; 

 and in the region of the Great Lakes Cap- 



