MECHANICS AND USEFUL ARTS. 69 



feet, are constructed of solid masonry. The gateways of the "bulk- 

 head, 13 in number, through which the water is let into the main ca- 

 nal, are 8 feet wide by 15 feet high, with double guard-gates, securely 

 put in. A gate-house is to be erected on the bulkhead, of sufficient 

 dimensions to cover the gates, and to contain the machinery for mov- 

 ing them. 



The gates were first closed on October 22d, and the water began 

 to run over the dam in 9 hours and 16 minutes from the time of clos- 

 ing. In the construction of the abutment, guard-gates, and lock-wall, 

 at the head of the canal, there have been used 10,000 perches of 

 stone (25 cubic feet to the perch). It is said that in peculiar states of 

 the air the roar of the water at this dam has been heard for the dis- 

 tance of 40 miles; and in Springfield, ten miles distant, it is heard 

 distinctly, and doors, whose latches allow them a little play, vibrate to 

 its concussions, at the rate of 128 vibrations per minute. 



To show the magnitude of the works of which this dam forms a 

 part, we may mention that 70,500 perches of masonry have been laid, 

 and 602,000 yards of earth and 50,000 yards of rock excavated, since 

 the work was first commenced. 



NEW SHIPPING-SIGNAL. 



A NEWXY invented apparatus for the prevention of collision at sea 

 during foggy and thick weather, \vhen lights and other methods now 

 in use are altogether unavailable, has been recently exhibited at 

 Lloyd's Rooms, in Liverpool. The machine is extremely portable, 

 occupying a space of about 2 feet square, and capable of being worked 

 by one man, who, turning a cog-wheel acting on a force-pump, pro- 

 duces a volume of sound that will penetrate to a divstance of several 

 miles. It was highly approved of by many merchants, captains, and 

 other nautical men, and it will answer, also, as a signal of distress for 

 vessels, when in danger on a lee-shore or elsewhere. 



INSTRUMENT FOR MEASURING A SHIP'S VELOCITY. 



MR. ARTHUR HUSTON, Bristol, Maine, has invented a very simple 

 machine, w y hich, on deck or cabin, or any convenient place, points to a 

 register marked with degrees to indicate the number of knots the ves- 

 sel is making per hour or half-hour. The principle of it consists in a 

 lever with a blade on its lower end, passing down on both sides of the 

 keel as a resisting medium to the water, which, by a graduated spring 

 on the upper edge of the lever, moves the lever backwards and for- 

 wards, according to the pressure of the water, and by having the 

 pointer on the upper end, the velocity of the vessel is thus indicated 

 on the dial. Two or more pointers may be piaeed in different parts 

 of the vessel, connected to the top of the lever by wires, to register 

 the velocity in different parts of the vessel at the same time, 



