sight of land, he could locate his vessel in such a position that his
balloons would float directly over the city and let fall a ton or two
of dynamite by means of a clock work attachment. The inventor had all
the minor details very plausibly worked out, such as locating by means
of pilot balloons the air currents at the proper height for the large
balloons, automatic arrangements for keeping the balloon at the proper
height after it was let go from the vessel, and so on. His scheme is
nothing but the idea of the drifting or current torpedo, which was so
popular during our war, transferred to the upper air. An automatic
flying machine would be one step farther than this inventor's idea,
and would be an exact parallel in the air to the much dreaded
locomotive water torpedo of to-day. There seems to be no limit to the
possibilities of high explosives when intelligently applied to the
warfare of the future, and the advantage will always be on the side of
the nation that is best prepared to use them.

* * * * *

THE MANUFACTURE AND USE OF PLASTER OF PARIS.

It has long been a familiar fact that gypsum yields on baking a
material which possesses the power of setting with water to a firm
mass, this setting being accomplished much more quickly than is the
case with mortar.

The explanation of the setting of plaster was first given by
Lavoisier, who pointed out that gypsum is an hydrated salt, and that
the set plaster is in fact gypsum reformed, the change brought about
by baking being merely loss of water of crystallization. The beds of
gypsum of most importance both formerly and at the present time in the
plaster manufacture occur in the neighborhood of Paris in the lower
tertiary formation. Different beds differ (1) in respect of character
and quantity of admixed materials and (2) in the structure of the
gypsum itself. With regard to the first point, some deposits contain a
notable proportion of carbonate of lime, a fact which under certain
circumstances may considerably influence the character of the plaster.
In the matter of structure two principal varieties occur (1) granular
and (2) fibrous. Further, hardness of the granular kind varies
considerably. These differences of structure in the original material
appear to exercise an influence on the properties of the plaster. Thus
according to Payen the plaster formed from the granular variety sets
more gradually than that derived fr