TRANSACTIONS OF SECTION A, 477 



TUESDAY, AUGUST 20, 1878. 



The Section divided itself into two departments for Astronomy and Physical 

 Science. — See p. 486 for the letter. 



The following Papers were read in the Department of Astronomy : — 



] . Report of the Committee on Luminous Meteors. By James Glaishek, F.R.S. 



See Reports, p. 258. 



Report of the Committee on the Tides in the English Channel. 

 See Reports, p. 217. 



3. On an Equatorial Mounting for a Three-Foot Reflector. 

 By the Earl op Rosse. 



4. On the Tides of the Southern Hemisphere and of the Mediterranean. 

 By Capt. Evans, R.N., and Sir William Thomson, LL.D. 



On the coasts of the British Islands and generally on the European coasts of the 

 North Atlantic and throughout the North Sea, the tides present in their main fea- 

 tures an exceptional simplicity, two almost equally high-waters and two almost 

 equally low-waters in the twenty-four hours, with the regular fortnightly inequality 

 of spring tides and neap tides due to the alternately conspiring and opposing actions 

 of the moon and sun, and with large irregular variations produced by wind. Care- 

 ful observation detects a small " diurnal " inequality (so called because it is due to 

 tidal constituents having periods approximately equal to twenty-four hours lunar or 

 solar), of which the most obvious manifestation is a difference at certain times of 

 the month and of the year between the heights of the two high-waters of the 

 twenty-four hours, and at intermediate times a difference between the heights of 

 the two low-waters. 



In the western part of the North Atlantic and in the North Sea this diurnal 

 inequality is so small in comparison with the familiar twelve-hourly or "semi- 

 diurnal " tides that it is practically disregarded, and its very existence is scarcely a 

 part of practical knowledge of the subject ; but it is not so in other seas. There 

 is probably no other gi-eat area of sea throughout which the diurnal tides are 

 practically imperceptible and the semi-diurnal tides alone practically perceptible. 

 In some places in the Pacific and in the China Sea it has long been remarked that 

 there is but one high water in the twenty-four hours at certain times of the month, 

 and in the Pacific, the China Sea, the Indian Ocean, the West Indies, and very 

 generally wherever tides are known at all practically, except on the ocean coasts 

 of Europe, they are known to be not " regular " according to the simple European 

 rule, but to be complicated by large differences between the heights of consecutive 

 high-waters and of consecutive low-waters, and by marked inequalities of the suc- 

 cessive intervals of time between high-water and low-water. 



On the coasts of the Mediterranean generally the tides are so small as to be not 



