B. TERRESTRIAL PHYSICS AND METEOROLOGY. 63 



Thomson, to substitute steel wire for the cord, was at first 

 quite unsatisfactory; but certain minor difficulties having 

 been obviated, the application of the new material proves to 

 be perfectly practicable. The wire is coiled around a wheel 

 one fathom in circumference, constructed of tin, and carrying 

 three miles of wire, No. 22 steel piano wire being found to be 

 the best, as this has twice the sustaining power of ordinary 

 iron wire. The difficulty of getting it of sufficient length was 

 overcome by Richard Johnston, of Manchester, who was able 

 to supply a homogeneous wire of three miles in length, 

 weighing but thirteen and a half pounds to the mile. This 

 is 0.03 of an inch in diameter, and bears a weight of 252 

 pounds, so that it will sustain twenty-one miles of its own 

 weight in the water. 



When the apparatus is arranged for use, a sounding-lead 

 of thirty pounds, with a brass tube in it for taking up a 

 specimen of the sea-bottom, is connected by means of thirty 

 fathoms of sounding-line to the end of the steel wire in ques- 

 tion, and at the point of union of the latter a lead weight of 

 three pounds is attached. This weight being directly at the 

 end of the steel wire, keeps it sufficiently stretched to pre- 

 vent any danger of kinking. An adjustable friction brake is 

 applied to the wheel, set to a pressure of about twenty 

 pounds, so that whenever the thirty-pound weight touches 

 the bottom the uncoiling of the wheel immediately ceases. 

 The wire, then, does not touch the bottom, and is kept taut 

 by its special lead. 



With the increasing depth the pressure of the brake is in- 

 creased proportionally, as it has been ascertained that for 

 each eighty fathoms of wire an addition of a pound of force 

 to the brake apparatus is required; and as each circumfer- 

 ence of the wheel represents a fathom for each eighty revolu- 

 tions, it is easy to calculate the force to be applied for a given 

 number of revolutions. In all cases it is necessary that the 

 wheel run out freely when held in the hand. 



Sir William Thomson made experiments in the Bay of 

 Biscay during the past summer, and found the apparatus to 

 be perfectly satisfactory. Once, w r hile expecting a depth of 

 1500 fathoms, the wire ran out 2500 fathoms without reach- 

 ing bottom. Continuing to pay out the line, with a brake 

 pressure of thirty-five pounds, greater and greater velocity 



