108 



Prof. J. Clerk Maxwell on a 



[Jan. 22, 



air was exhausted and the liquid forced up the tube by the pressure of 

 the atmosphere, to a height of 3235 71 inches, being equivalent to 30 

 inches of mercury, the Kew standard at the time reading 30'3 inches. 

 The plug in the cistern was now screwed in its place, to support the 

 column, while the air was admitted at the top and the air-pump con- 

 nexions removed ; a sufficient quantity of glycerine to fill the tube 

 was then poured in and the india-rubber stopper inserted. The screw 

 plug being removed for a few seconds to allow the column to fall an 

 inch or two and then replaced, the instrument was allowed to remain 

 until the liquid in it was completely exhausted of its air, which rose 

 slowly to the surface into the Torricellian vacuum above : then the 

 india-rubber stopper was again withdrawn and the tube finally filled 

 up with glycerine, which had been previously exhausted of air under 

 the air-pump receiver : the stopper was now replaced and the cistern 

 plug finally removed, when the column gradually fell until balanced 

 by the pressure of the atmosphere, leaving a small quantity of 

 glycerine in the cup above the stopper, a plate glass cover being- 

 placed on the top to keep out dust. The barometer was now complete 

 and it has since continued in operation. Whether it is to be of any 

 scientific or practical value will be proved by the observations which 

 are now being regularly taken under the superintendence of Mr. Whip- 

 ple, the Superintendent of the Kew Observatory, to whom I am under 

 many obligations for his kind and courteous assistance during the 

 progress of the work. When the observations are completed I shall 

 ask the honour of submitting them to the Royal Society. 



IV. " On a Possible Mode of Detecting a Motion of the Solar 

 System through the Lnminiferons Ether." By the late Pro- 

 fessor J. Clerk Maxwell, F.R.S. In a Letter to Mr. D. P. 

 Todd, of the Nautical Almanac Office, Washington, U.S. 

 Communicated by Professor Stokes, Sec. U.S. Received 

 January 7, 1880. 



Mr. Todd has been so good as to communicate to me a copy of the 

 subjoined letter, and has kindly permitted me to make any use of it. 



As the notice referred to by Maxwell in the " Encyclopaedia 

 Britannica" is very brief, being confined to a single sentence, and as 

 the subject is one of great interest, I have thought it best to com- 

 municate the letter to the Royal Society. 



From the researches of Mr. Hnggins on the radial component of 

 the relative velocity of our sun and certain stars, the coefficient of the 

 inequality which we might expect as not unlikely would be only some- 

 thing comparable with half a second of time. This, no doubt, would 

 be a very delicate matter to determine. Still, for anything we know 



