Ixxvi KEPORT — 18/6, 



the action of licat or light; and the question M'hich of these forces is the 

 efficient agent in causing the motion of the tiny disks in Crookes's radiometer 

 has given rise to a good deal of discussion. The answer to this question in- 

 volves the same principles as those bj'which the image traced on the dnguerrco- 

 type i)late, or the decomposition of carbonic acid by the leaves of plants, is 

 referred to the action of light and not of heat ; and applying these principles 

 to the experiments made Tvith the radiometer, the weight of evidence appears 

 to be in favour of the view that the repulsion of the blaclvcncd surfaces of 

 the disks is due to a thermal reaction occurring in a highly rarefied mcdiimi. 

 I have myself had the pleasure of -witnessiug many of Mr. Crookes's experi- 

 ments, and I cannot sufficientljf express my admiration of the care and skill 

 "with which ho has pursued this investigation. The remarkable repulsions 

 he has observed in the most perfect vacua hitherto attained are interesting, 

 not only as having led to the construction of a beautiful instrument, but as 

 being likely, -when the subject is fully investigated, to give valuable data for 

 the theory of molecular actions. 



A singular property of light, discovered a short time ago by Mr. Willoughby 

 Smith, is its power of diminishing the electrical resistance of the element 

 selenium. This property has been ascertained to belong chiefly to the luminous 

 rays on the red side of the spectrum, being nearly absent in the violet or 

 more refrangible rays and also in heat-rays of low refrangibility. The 

 recent experiments of Prof. W. G. Adams have fully established the accuracy 

 of the remarkable observation, first made by Lord llosse, that the action ap- 

 peared to vary inversely as the simple distance of the illuminating source. 



Switzerland sent, some years ago, as its representative to this country the 

 celebrated Do la Rive, whose scientific life formed lately the subject of an 

 eloquent f'lV/e from the pen of M. Dumas. On this occasion we have to 

 welcome, in Oeneral Menabrea, a distinguished representative both of the 

 kingdom of Italj- and of Italian science. His great work on the determina- 

 tion of the pressures and tensions in an elastic system is of too abstruse a 

 character to be discussed in this address; but tlie principle it contains may 

 be briefly stated in the following words : — " When any elastic sj-stem places 

 itself in equilibrium under the action of external forces, the work developed 

 by the internal forces is a minimum." General Menabrea has, however, other 

 and special claims upon us here, as the friend to whom Babbage entrusted the 

 task of making known to the world the principles of his analytical machine 

 ■ — a gigantic conception, the efibrt to realize which it is known was one of 

 the chief objects of Babbage's later life. The latest development of this con- 

 ception is to be found in the mechanical integrator of Trof. J. Thomson, in 

 M-hich motion is transmitted, according to a new kinematic princiiJe, from a 

 disk or cone to a cylinder through the intervention of a loose baU, and in 

 Sir ^\. Thomson's machine for the mechanical integration of differential 

 equations of the second order. In the exquisite tidal machine of the latter 



