314 



Rev. S. J. Perry on Magnetic 



[Nov. 16, 



When either of these four radiometers is heated by a hot shade or 

 plunged into hot water, rotation is always produced in the opposite 

 direction to that caused by the light. On removing the source of heat the 

 motion rapidly stops, and then commences in the opposite direction (^. e. 

 as it would rotate under the influence of light), the rotation continuing 

 as long as the fly is cooling. Chilling one of these radiometers with ether 

 has the opposite action to exposing it to dark heat. 



The vanes of radiometers have also been formed of metal cones, and 

 of cups and cones of plain mica, roasted mica, pith, paper, &c. ; and they 

 have been used either plain or blacked on one or both surfaces. These 

 have also been balanced against each other, and against metal plates, cups, 

 and cones. The results are of considerable interest, but too complicated to 

 explain without great expenditure of time and numerous diagrams. The 

 broad facts are contained in the above selections from my experiments. 



The action of light on the cup-shaped vanes of a radiometer probably 

 requires more experimental investigation before it can be properly under- 

 stood. Some of the phenomena may be explained on the assumption that 

 the molecular pressure acts chiefly in a direction normal to the surface 

 of the vanes. A convex surface would therefore cause greater pressure 

 to be exerted between itself and the bounding surface of glass than would 

 a concave surface. In this way the behaviour of the cup-shaped radio- 

 meter with both surfaces bright. No. 1035, can be understood, and per- 

 haps also that of Nos. 1038 and 1039. It would not be difficult to test 

 this view experimentally, by placing a small mica screen in the focus of a 

 concave cup where the molecular force should be concentrated. But it is 

 not easy to see how such an hypothesis can explain the behaviour of 

 No. 1037, where the action of the bright convex surface more than over- 

 comes the superior absorptive and radiating power of the concave black 

 surface ; and the explanation entirely fails to account for the powerful 

 attraction which a lighted candle is seen to exert on the concave surfaces 

 in Nos. 1035, 1037, and 1039. 



II. '^Magnetic Observations made at Stonyhurst College Obser- 

 vatory from April 1870 to March 1876.'^ By the Rev. S. 

 J. Perry, S.J., F.R.S. Received July 31, 1876. 



A double series of magnetic observations are being carried on conti- 

 nuously at this Observatory. The monthly determinations of the absolute 

 elements date from the year 1863, but the uninterrupted photographic 

 record of the variations of the Declination and of the components of the 

 Intensity was only commenced in 1867. The photographic curves of the 

 Declination and Horizontal Force have all been measured, and are in course 

 of reduction ; and the Yertical-Force curves will soon be taken in hand. 

 The results of the first seven years' observations of the Dip and Hori- 

 zontal Porce appeared in the Philosophical Transactions for 1871, and the 



