128 Dr. Tyndall on Molecular Influences. 



great care is necessary. We must not use the bellows in some 

 experiments and neglect the use of it in others ; and if the aether 

 be applied once, it must be applied throughout the entire series 

 of experiments. It must continue to act for the same time, and 

 the same quantity should be applied in all cases. Of course such 

 precautions are only necessary when great accuracy is required, 

 but here they are absolutely necessary. Judging from the de- 

 scription, the mode of experiment may appear complicated, but 

 in reality it is not so. A single experimenter has the most com- 

 plete command over the entire arrangement. The wires from 

 the small galvanic battery (four of Bunsen's cells) remain undis- 

 turbed from day to day ; all that is to be done is to connect the 

 battery with them, and every thing is ready for experiment. 



There are in wood three lines at right angles to each other, 

 which the mere inspection of the substance enables us to fix 

 upon as the necessary resultants of molecular action : the first 

 line is parallel to the fibre ; the second is perpendicular to the 

 fibre, and to the ligneous layers which indicate the annual growth 

 of the tree; while the third is perpendicular to the fibre and 

 parallel, or rather tangent to the layers. From each of a number 

 of trees a cube was cut, so that every two opposite faces were 

 parallel to one of the above lines. Thus, two faces were parallel 

 to the ligneous layers, two perpendicular to them, while the re- 

 maining two were perpendicular to the fibre. It was proposed 

 to examine the velocity of calorific transmission through the 

 mass in these three directions. It may be remarked that the 

 cubes were fair average specimens of the woods, and were in all 

 cases well-seasoned and dry. 



The cube was first placed upon its four supports abed, so that 

 the line of flux from m! to m was parallel to the fibre, and the 

 deflection produced by the heat transmitted in sixty seconds was 

 observed. The position of the cube was then changed so that 

 its fibre stood vertical, the line of flux from m 1 to m being per- 

 pendicular to the fibre and parallel to the ligneous layers ; the 

 deflection produced by a minute's action in this case was also 

 determined. Finally, the cube was turned 90° round, its fibre 

 being still vertical, so that the line of flux was perpendicular to 

 both fibre and layers, and the consequent deflection was observed. 

 In the comparison of these two latter directions the chief delicacy 

 of manipulation is necessary. It requires but a rough experi- 

 ment to demonstrate the superior velocity of propagation along 

 the fibre, but the velocities in all directions perpendicular to the 

 fibre are so nearly equal that it is only by the greatest care and, 

 in the majority of cases, by numerous experiments, that a differ- 

 ential action can be securely established. 



The following tabic contains the results of the inquiry ; it will 

 explain itself. 



