ADDRESS. Ixxi 



rclical grounds by Hansen and Leverrier, -wiiou Foucault, in 1802, decided 

 the question by an experiment of extraordinary delicacy. Taking advantage 

 of the revolving-mirror, with which Wheatstono had some time before 

 enriched the physical sciences, Foucault succeeded in measuring the absolute 

 velocity of light in space by experiments on a beam of light, reflected 

 backwards and forwards, within a tube little more than thrrtcen feet in 

 length. Combining the result thus obtained with what is called by astrono- 

 mers the constant of aberration, Foucault calculated the distance of the earth 

 from the sun, and found it to be one thirtieth part, or about 3,000,000 miles, 

 less than the commonly received number. This conclusion has lately been 

 confirmed by 31. Cornu, from a new determination he has made of the 

 velocity of light according to the method of Fizeau ; and in complete accord- 

 ance with these results are the investigations of Leverrier, founded on a 

 comparison with theory of the observed motions of the sun and of the 

 planets Tonus and Mars. It remains to be seen whether the recent obser- 

 vations of the transit of Yenus, when reduced, will be sufficiently concordant 

 to fix with even greater precision the true distance of the earth from the 

 sun. 



In this brief reference to one of the finest results of modern science, 

 I have mentioned a great name whose loss England has recently had to 

 deplore, and in connexion with it the name of an illustrious physicist whose 

 premature death deprived France, a few years ago, of one of her brightest 

 ornaments — Vhcatstone and Foucault, over to be remembered for their 

 marvellous power of eliciting, like Galileo and Newton, from familiar 

 phenomena the highest truths of nature ! 



The discovery of Huggins that some of the fixed stars are moving to- 

 wards and others receding from our system, has been fully confirmed by 

 a careful scries of observations lately made by Mr. Christie in the Observa- 

 tory of Greenwich. Mr. Huggins has not been able to discover any indications 

 of a proper motion in the nebulas ; but this may arise from the motion of 

 translation being less than the method would discover. Few achievements 

 iu the history of science are more \^-onderful than the measurement of the 

 proper motions of the fixed stars, from observing the relative position of two 

 delicate lines of light in the field of the telescope. 



The observation of the American astronomer Young, that bright lines, 

 corresponding to the ordinary lines of Fraunhofer reversed, may be seen in 

 the lower strata of the solar atmosphere for a few moments during a total 

 eclipse, has been confirmed by Mr. Stone, on the occasion of the total eclipse 

 of tho sun which occurred some time ago in South Africa. In the outer 

 corona, or higher regions of the sun's atmosphere, a single green line only 

 was seen, the same which had been already described by Y^oung. 



I can here refer only in general terms to the observations of lloscoe and 

 Schuster on the absorption-bands of potassium and sodium, and to the in- 



