xi 



tides in the Thames, at Ipswich, Southampton, the coast of Ireland, 

 * and, later on, the tides at Malta. A Royal Medal was adjudged to 

 him by the Royal Society in 1845 for his inquiry into the laws of 

 the tides on the coast of Ireland. His chief work on this subject is 

 his essay on " Tides and Waves," printed in the ' Encyclopaedia 

 Metropolitans' Taking the usual division of the theory into three 

 parts, viz., the equilibrium theory, that of ocean tides, and that of 

 river tides, we may ascribe the initial steps in the first to Newton and 

 Bernoulli, those in the second to Laplace, while the last may be said 

 to have begun with Airy. This important paper, perhaps because it 

 had not been published by any learned society, did nob attract the ' 

 attention it deserved on the Continent, but its merits could not remain 

 unnoticed, and in 1875 the section on river tides was translated and 

 printed in ' Liouville's Journal.' In this section he discusses the broken 

 water seen on the edge of a shoal, why the rise of tide occupies less time 

 than the fall, the solitary wave, the breaking of waves, the effect of 

 the wind, tidal waves, the effect of friction, the form of the wave in 

 broad channels with shallow sides, and other interesting questions. 

 When M. Delaunay, in 1866, suggested that a portion of the apparent 

 acceleration of the Moon was due to a real retardation of the rotation 

 of the Earth caused by tidal friction, Mr. Airy gave a general ex- 

 planation, founded on the theory of river tides. He discovered two 

 terms of the second order in his equations whose general effect was 

 to produce a constant acceleration of the waters in the direction of 

 the Moon's apparent diurnal course. He therefore gave his entire 

 assent to the views of Delaunay on the existence of one real cause for 

 the retardation of the Earth's rotation. Other causes of retardation 

 have been discovered by mathematicians since then, but these, of 

 course, lie outside the scope of the present sketch. 



In the summer of 1844 the arc of longitude between Greenwich and 

 the island of Valentia was measured. As an intermediate station, 

 the longitude of Kingstown was also determined. The difference of 

 longitude was found by making thirty pocket chronometers travel 

 from Greenwich to Kingstown and back again several times ; the 

 difference between Kingstown and Valentia being found in a similar 

 manner. The differences in longitude having been found, the next 

 step was to compare the results with the data of the trigonometrical 

 survey, and to see how far they agreed with the best existing deter- 

 mination of the figure of the Earth. The triangulation was then only 

 partially completed, but enough had been done to enable Mr. Airy to 

 arrive at the result that in the latitude of 51° 40' the length of 1" 

 in an arc perpendicular to the meridian is 101*6499 feet in terms of 

 the standard bar of the Ordnance Survey. In the same year Struve 

 determined the difference of the longitudes of Altona and Greenwich 

 by the transmission of forty-two chronometers across the German Sea 



