SCIENTIFIC SUMMAEY. 
115 
valve of the reservoir, admitting a supply of air equivalent to that respired 
hy the diver. And so, also, in passing from a less to a greater depth, the 
valve will be opened till the pressure of the air in the chamber is in equi- 
librium with the pressure of the water by which the diver is surrounded. 
The valve of the reservoir r is a conical metallic spindle valve ; that at 
s, for permitting the passage of the respired air, is .a simple aperture covered 
with india-rubber, in the same manner as an air-pump valve. 
Resistance to Rupture of Hydrostatic Cylinders. — Hitherto it has been 
assumed that the cause of rupture of cast-iron cylinders to internal bursting 
pressure resulted simply from pressure on their interior walls ; but Mr. St. 
John Y. Day, C.E., has called attention to some observations made by Messrs. 
Weems, which seem to show that in press cylinders the water obtains entrance 
into the intermolecular spaces of the cast iron, in effect slicing off a ring of 
the metal, and proportionately reducing its resistance to internal fluid pressure. 
It is a known fact, that under comparatively moderate pressures water finds 
its way completely through cast iron, diffusing itself over the external surface 
as a fine dew. Taking it for granted, then, that this interstitial permeation of the 
fluid diminishes the tenacity of the material, Messrs. Weems have sought to 
prevent its entrance by lining the cylinders with a material of great density 
and high impermeability. Hitherto, a press cylinder, not strengthened ex- 
ternally, having a ram of 24 inches diameter, could not be relied on at a 
pressure above 2^ tons per square inch. A cylinder of this diameter bored 
out and lined with copper by Messrs. Weems, is now working at a constant 
stress of 3 '5 tons per square inch. — Artizan, Nov. 1865. 
MICROSCOPY. 
Illuminator for Opaque Objects under High Powers. — An ingenious instru- 
ment for effecting this purpose was described by Mr. Smith, of Kenyon 
College, U.S., in the September number of Sillimari's American Journal 
This, with a very slight but advantageous modification, has now been manu- 
factured and patented by Messrs. Powell & Lealand. In it the object-glass 
is made its own illuminator. The woodcut beneath will explain how this is 
achieved. A short brass tube screws on to the end of the body of the micro- 
scope, and carries the objective at its other extremity. It is perforated by a 
small aperture, and has within it, and opposite To this aperture, a plate of 
clear glass, inclined at such an angle, that light entering the aperture, and 
falling upon the plate, is thrown down through the object-glass, and upon the 
object. The light is derived from a lamp placed near the aperture, and having 
its rays of light collected by a bull’s-eye condenser. In Mr. Smith’s arrange- 
ment, a silver mirror was employed ; but as this cut off half the pencil of 
rays proceeding from the object, Messrs. Powell & Lealand conceived the 
substitution of a plate of clear glass. This instrument is certainly the most 
perfect contrivance imaginable. It can be used with power as high as the 
T Vth inch, and from what we have seen of its work with a T V inch, we can 
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