The annexed figure represents a watch alarum that is sold in the shops of London, and which we have seen perform with con siderable accuracy. The expense of it is only seven shillings. a a is a turned mahogany stand ; b the watch laid in a velvet cush ioned cavity adapted to receive it, and placed in such a position, that the hour at which a person may wish to rise, shall be placed opposite to a fixed index c. A fine line, consisting of a single horse hair, with a loop at the end of it, is then laid into the notch of a guide piece d, and the loop is then slipped over the hour hand of the watch. At e is a light ivory lever; to this the horse hair is tied about midway of its length, with a weight f suspended to its lowest end. The bell g, fixed on its steel spring, is then brough. into the position shown by the line h, the extremity of which line is provided with a brass wire hook, then passed over the extremity of the lever, and put on to the upright pin 1. When, by the process of time, the hour hand has amved at the period proposed, which is opposite to the point of the index, the horse hair slips from it, the little weight thereby becomes unsupported, pulls down the ivory lever, raising the hook of the pin, which, releasing the spring, sets the bell ringing.
The periodical journals a few years ago abounded with plans of simple alarms; and any person having a taste for such trifles in mechanics might easily multiply them, as the materials as well as the arrangements of parts may be almost in finitely varied. Sand, passing through a minute perforation, (as in the hour-glass,) and charging a receptacle, whose weight in due time gave motion to a bell, was a common expedient. The substances used for domestic light have also been called into use, to show by their uniform decrease of quantity the time passed, and by their decrease of weight in consequence allow the reaction of a constant force to give motion to an alarum. The most perfect and elegant piece of mechanism for this purpose, is Berrollas's patent watch alarum, which we have fully described under the head HoaoLooy.