THE RADIOMETER. 37 



really it should be just the reverse, and practically for accuracy the 

 larger the ship the smaller should be the compass. If I had my will 

 a ship like the Great Eastern instead of having, (as she originally had,) 

 a compass with a needle fourteen inches long, should have one of 

 three or four inches, as being more conducive to her safety. It would 

 take too long to enter upon either the theoretical or mathematical con- 

 siderations bearing on this subject ; but they are perfectly sound, and 

 my opinion is confirmed by the views that Sir W. Thomson has ex- 

 pressed in introducing these cards to your notice. 



The PRESIDENT : Dr. Wartmann, Professor of Natural Philosophy 

 at the University of Geneva, will now give you an account of his expe- 

 riments with the Radiometer. 



M. ELIE WARTMANN, Professor of Natural Philosophy in the Uni- 

 versity of Geneva : In the address which he delivered at the opening 

 of this conference, Mr. W. Spottiswoodc recalled to mind the invention 

 of the air-pump by Otto von Guericke. As soon as scientific men 

 knew how to produce a vacuum, they set to work to observe what 

 phenomena might manifest themselves in it. About the beginning of 

 the i Qth century, Sir Humphrey Davy proved that it is permeable to 

 heat, which influenced a thermometer placed in the middle of the 

 chamber of a barometer. Three years ago, Mr. W. Crookes, F.R.S., 

 excited general attention by the curious instrument to which he has 

 given the name of the Radiometer. It is known that it consists of a 

 delicate mill, revolving freely in a glass vessel, in which the best 

 possible vacuum has been produced. This mill has four arms 

 supporting as many paddles (palettes), each one of which is painted 

 black on one side and white on the other. 



This pretty instrument has, in England, often been called the " Light 

 Mill," and many persons are endeavouring to make use of it as a 

 photometer to determine the intensity of the light produced by gas 

 and by other means. This is the result of an erroneous impression which 

 ought to be removed. It is by no means light, as such, which cause3 

 the instrument to revolve, but solely heat, either luminous or obscure. 

 By exposing one of the blackened paddles to the focus of a large lens, or 

 of a concave mirror, the light of the full moon passing the meridian 

 may be concentrated on it without producing the slightest motion. 

 And it can likewise be proved that when revolving under the imluenco 



