of Edinburgh, Session 1873-74 
363 
4. On a New Form of Mariner's Compass. 
By Sir William Thomson. 
Monday , 6th April 1874. 
Sir WILLIAM THOMSON, President, in the Chair. 
The following Communications were read : — 
I. Further Note on Spectra under exceedingly Small 
Pressures. By Professor Tait and James Dewar, Esq. 
2. On the After-glow of Cooling Iron at a Dull-Red 
Heat. By George Forbes, Esq. 
The facts to be explained were observed by Messrs Gore and 
Barrett, and were described by the latter gentleman in the “ Philo- 
sophical Magazine ” for 1873. 
The experiments are performed on an iron or steel wire of no 
great thickness. When this is heated to an intense white heat 
and allowed to cool, the following facts appear at the instant it 
has cooled down to a dull-red heat : — 
1. The wire expands for an instant, and then continues its 
normal contraction. 
2. The glow from the wire is at the same instant seen to increase. 
3. The temperature of the air round the wire is at the same 
instant increased. 
4. The same facts are seen when the wire is in an atmosphere 
of hydrogen. 
5. If the wire be very thin the cooling is so rapid that the 
effects are not observed. 
6. If the iron be massive the effects are not observed, 
7. If the wire be not originally heated up to an intense white 
heat the effects are not observed. 
That iron should increase its temperature at a dull- red heat 
VOL. VIII. 
