1 74 Mr. W. E. Wilson. Effect of Pressure on the [May 30, 



that practically all compound mechanisms contain a continuous 

 mechanism A, of not more than four links, from which definiteness of 

 relative motion of all the other links is derived. Any two links 

 of A in their exact length, or longer or shorter, may be adapted to 

 form with two new links a second mechanism B, and any two of A or 

 B, or one of A and one of B, may be adapted to form with two still 

 further added links a third mechanism C, and so on. In this way a 

 definiteness of relative motion of many links in a compound 

 mechanism is derived. The notation lends itself to a clear exhibition 

 of the manner in which two or more simple mechanisms are associated 

 together, and the compound mechanism built up. 



III. " On the Effect of Pressure of the Surrounding Gas on the 

 Temperature of the Crater of an Electric Arc Light. 

 Preliminary Notes of Observations made at Daramona, 

 Streete. co. Westmeath." By W. E. WILSON. Communi- 

 cated by Professor FITZGERALD, F.R.S. Received April 

 25, 1895. 



Of late years it has often been assumed that the temperature of 

 the crater forming the positive pole of the electric arc is that of the 

 boiling of carbon. The most modern determinations give this point 

 as about 3300 3500 C. 



Solar physicists have thought that the photosphere of the sun 

 consists of a layer of clouds formed of particles of solid carbon. As 

 the temperature of these clouds is certainly not below 8000 C., it 

 seems very difficult to explain how carbon can be boiling in the arc 

 at 3500 and yet remain in the solid form in the sun at 8000. Pres- 

 sure in the solar atmosphere seemed to be the most likely cause of 

 this, and yet, from other physical reasons, this seemed not probable. 



In order to investigate whether increased pressure in the gas sur- 

 rounding an electric arc would raise the temperature of the crater, I 

 had an apparatus made by the Cambridge Instrument Company. It 

 consists of a strong cast-iron box, which was tested by hydraulic 

 pressure to 2000 Ibs. per square inch. In the following plan, A is 

 the box, B and C are the two carbon poles enclosed in steel tubes. 

 The negative carbon was kept in position against a copper ring by a 

 spiral spring behind it. The positive carbon was hand fed by a 

 friction roller, which was moved by a handle F outside the box. A 

 steel tube H was screwed into the box at such an angle that, by 

 looking down it, we could see well into the crater of the positive pole. 

 The end of this tube is closed by a glass lens, which formed an image 

 of the crater at a distance of 80 cm. 



