189
When the machine is in operation, the pulley line, or endless rope, runs freely round with the drum and pulley wheel, but the dynamometer wheel being held fast by the small cord attached to the spring balance, the friction of the turns of the rope running round the wheel expends itself in bringing a strain on the balance, the index of which registers the number of pounds of that strain; it is needless to say the strain is in proportion to the amount of weight on the pendant.
The piano wire No. 22 in size, weighs, in water, about 12 lbs. to the statute mile, and will bear a strain of from 200 lbs to 230 lbs. The wire comes in lengths of from 200 fathoms to about 400 fathoms, and has to be spliced to make it available for sounding purposes. The splices are made some three feet in length, the parts being put together with a long jawed twist, and the ends and three or four intermediate points secured with solder. The whole length of the splice is then served with fine waxed thread and the splice is complete. In no case have the splices drawn or broken. To keep the wire free from rust, it is kept at all times when not in immediate use, in tank containing a solution of caustic soda. This protects the wire completely, and the piece before the Society this evening has been in use ten months.
To the outer end of the wire is attached a light galvanized iron ring, or rope grummet, to which is made fast some 25 fathoms of cord or Albacore line; to the other end of this line is attached the apparatus for the detachable sinker and specimen cup. The purpose of this line is to prevent the wire from coming into contact with the bottom, for if that were allowed, the wire being stiff and elastic, would be apt to fly upward, kink, and break.
The sinkers used are 8-inch shot with holes bored through their centres 214 inches, and 234 inches in diameter, through which the Brooke detaching rod and the specimen cylinders are passed; their weight is 55 lbs. and 51 lbs. Sir William Thomson used a lead sinker weighing 30 lbs. which he hauled back with the wire, but that plan put too much stress on the machine in reeling in, and the heavier
