The greatest security against a door being opened from the outside, is undoubtedly obtained by means of bolts operated only from the inside of the door.

Fig. 40X.   Barrel Bolt, Bent Staple.

Fig. 40X. - Barrel Bolt, Bent Staple.

Fig. 401a.   Light Square Bolt.

Fig. 401a. - Light Square Bolt.

Fig. 402   Square Case Bolt.

Fig. 402 - Square Case Bolt.

The simplest bolts are those which are made to screw to the inside of the door, and of these the most common is the barrel bolt, Fig. 401. The common barrel bolt, however, has a plain flat staple plate, and if a staple plate like that shown in the illustration (which is obviously much stronger) is desired, a "bent staple plate" should be specified. When made of wrought steel, the bolt shown in Fig. 401 is believed to be the strongest bolt made. Fig. 401a shows a light square bolt, and Fig. 402, a "square case" bolt, which differs from the square bolt in not having as long a slide, and the end of the case is closed. Square bolts of these patterns are not usually over 4 inches long, while barrel bolts are made 3, 4, 5, 6 and 8 inches long. Small barrel and square case bolts may be obtained in solid bronze. Cast iron bolts are not very reliable, as being brittle, the bolt or case may be broken.

Bolts of the above description, however, do not have a very neat appearance in nicely finished rooms, and hence in such places mortised bolts are preferred. The neatest mortise bolt is probably the knob bolt shown in Fig. 375, or an extra bolt in the lock, as in the three-bolt chamber door lock.

FIG. 403   Gem Bolt.

FIG. 403 - Gem Bolt.

Fig. 404.   Chain Bolt.

Fig. 404. - Chain Bolt.

When the door is already fitted with a common knob lock, a cylinder mortise bolt, of which the well-known Gem bolt, Fig. 403, is one of the best examples, may be used to advantage. This bolt is very simple in its mechanism, as shown by the section drawing, and when fully thrown the bolt cannot be pushed back.

Chain Bolts. - These are sometimes used on front doors of dwellings ; they permit the door to be opened a few inches to see who is at the door, while preventing it from being further opened.

Fig. 404 illustrates the typical chain bolt, which consists of a slotted plate to go on the face of the door, and a chain secured to the door jamb with a dog on the end of the chain which will slide freely in the slot of the plate. A holder is provided to which the chain can be attached when not in use. There are many varieties of these fasteners, all based on the same principle. They are generally of brass or bronze, finished to correspond with the rest of the trimmings.