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Mechanical Science. 373 



parabolic spindles, sharp at the ends, of about an inch in 

 length and -^^ in diameter. In order to ensure perpendicularity 

 of the axes, before such a ball was hermetically sealed, a small 

 globule of mercury was introduced, which effectually answered 

 the purpose. 



The mode of observation was as follows : — A jar with distilled 

 water, thermometers and a bulb being arranged, the tempera- 

 ture being so low that the ball remained at the bottom of the 

 water, was carefully watched with alarge lens until the ball quitted 

 the bottom, and at this moment the thermometers Avere noted. 

 When the ball had risen a littlt, a small rod was cautiously let 

 down, and, without agitating the water, gently made to touch the 

 ball; it descended, but instantly rose -.this is avery delicate part of 

 the experiment, and if overdone loses its effect. It was repeated 

 frequently, and the ball re-ascended each time with accelerated 

 velocity. The thermometer indicating an increasing tempera- 

 ture, the ball finally became stationary at the surface ; from time 

 to time it was touched as before, but, as the temperature rose, 

 the tendency of the ball to ascend, judging by the velocity 

 with which it did so, each time diminished. Its upper ex- 

 tremity, by degrees seemed to press more feebly on the surface 

 of the water, till at last a fine thread of separation became 

 visible. The degree by the thermometers was again marked, 

 and, as they continued slowly to rise, the ball gradually fell to 

 the bottom of the jar. The intermediate point, between the 

 two points noted, was then ascertained, and considered as the 

 point of maximum density of the water. It appeared, from 

 all the experiments, to be a little above 42° ; and, from one 

 experiment, as before mentioned, to be 42,3°. — Ann. Phil. 

 N.S. V. 



11. Tenacity of Iron Wire. — At page 136, an account is given 

 of an economical wire suspension-bridge erected at Annonay, by 

 M. Seguin. It was expected that the difference of temperature 

 at different seasons would influence the strength of this and 

 similar bridges, and render it weaker at one time than another. 

 M. Dufour has, therefore, undertaken some experiments, with 

 a view of ascertaining any change in tenacity dependent upon 

 such alteration of temperature. Some iron wire was procured, 

 ■^ of an inch in diameter, and the weight required to break it 

 ascertained from the mean of several experiments. A portion was 

 then passed through a hollow vessel, filled with a frigorific mix- 

 ture, which lowered the temperature to — 8° F. In three experi- 

 ments, in which wires, thus circumstanced, were broken by 

 weights applied to them, the separation took place out of 

 the vessel, and the weight required was the same as before. 

 The vessel was then filled with boiling water, and the wire 

 passing through it tried as before. It broke once in the vessel, 

 and once out of the vessel, the latter by the smaller weight. 



