BIRD HOUSES AND HOW TO BUILD THEM. 
11 
through it runs a cord by which tho house is raised and lowered. 
The floors are all removable by lifting up. When the house is out of 
contact with the roof all of tho entrances are closed by gates actuated 
by springs, the gates moving upward to close, and being kept dowji 
and open by pressure against the roof. By means of this device spai- 
rows may be kept out of 
the house until martins are 
due to arrive, or if they get 
m when the house is open 
they can be trapped by sud- 
deifly lowering it. The pole 
shown here is made from 
hardwood boards put to¬ 
gether with screws. The 
concrete base has a core 
of 2-inch iron pipe which 
extends upward far enough 
to make a firm connection 
with the upper part on 
which the house shdes. A 
heavy weight is employed to hold the house hard against the roof. 
By passing the cord around the hook of the weight exactly as shown 
in figui'e 39 and pulling it upward until the weight is clear of the 
ground, it can easily be held mthout slipping while a more secure 
knot can be tied. A hook less wearing to the cord and fully as serv¬ 
iceable may be made from an acute natural crotch of oak or other 
Fig. 30.—Sections of house shown in figure 29. Upper floor 
removable. 
r-/ s-£ 
Fig, 31.—Lumber diagrams for house shown in figure 20. Thickness of boards J inch. 
hardwood instead of iron. Where this house is exposed to strong 
winds it may be advisable to attach guy wires to corners of the roof. 
The pole may be made of a single piece of 4-inch galvanized pipe, j=iet 
in a concrete base. In this case the house should be a cylmder and 
the roof a cone. 
