PROJECTIONS. 37 



working in front of the lens, the same as with the 

 porte lumiere. • 



The following is a list of apparatus and of experi- 

 ments which are suitable for such projection : Equil- 

 ibrium of the same liquid in several communicating 

 vessels; equilibrium of different liquids in communi- 

 cating vessels ; cartesian diver ; the hydrometer ; cap- 

 illarity j diffusion of gases ; Torricelli's experiment ; 

 Mariotte's law ; the manometer ; Sprengel's air pump ; 

 fountain in vacuo ; the siphon ; the pyrometer ; the in- 

 fluence of pressure upon the boiling point ; M. Des- 

 pretz's experiment on the conductivity of solids ; con- 

 vection ; the thermo-pile ; umbra and penumbra ; action 

 of magnets ; attraction and repulsion from electrical ex- 

 citation. Natural history specimens, such as birds, rats, 

 mice, squirrels, frogs, toads, live fishes, if in a tank with 

 transparent sides ; leaves of trees, ferns, etc. ; well-de- 

 fined crystals, such as quartz, feldspar, mica, pyrite -, 

 diagrams on glass of machinery, as the steam engine — 

 these diagrams can be drawn a foot square or more ; 

 silhouettes, etc., etc., are all available with this method. 

 There is an advantage in this plan, when it is at all 

 applicable, that will commend itself to every one, 

 namely, it is available at any point between the focus 

 and the screen, hence it will only be necessary to place 

 the object in the path of the rays to the screen at such 

 a point as will be convenient and will make the shadow 

 sufficiently large. The instructor can stand by the ob- 

 ject, and with a pointer like a pencil call attention to 

 any particular part. And again, the field is so large 

 that several objects can be in it at a time, if need be, 

 for comparison, such for instance as leaves of several 

 species of oaks or maples, or a range of capillary tubes 

 of various diameters. 



