Gravitation 



FIG. 6. An air pump. 



Experiment 5. Put a cork into an empty bottle. Do 

 not use a new cork, but one that has been fitted into the 

 bottle many times and has become shaped to the neck. 

 Press the cork in rather firmly, so that it is air-tight, but do 

 not jam it in. Set the bottle on the plate of the air pump, 

 put the bell jar over it, and pump the air out of the jar. 

 What makes the cork fly out of the bottle? What was 

 really in the " empty " bottle? Why could it not push the 

 cork out until you had pumped the air out of the jar ? 



Experiment 6. Wax the rims of the two Magdeburg hemi- 

 spheres (see Fig. 7). Screw the lower section into the hole 

 in the plate of the air pump. Be sure that the stop valve 

 in the neck of the hemisphere is open. (The little handle 

 should be vertical.) Fit the other section on to the first, 

 and pump out as much air as you can. Close the stop valve. 

 Unscrew the hemispheres from the air pump. Try to pull 

 them apart pull straight out, taking care not to slide the 

 parts. If you wish, let some one else take one handle, and 

 see if the two of you can pull it apart. 



