xvi 



tion by their author, published in a collected form. This treatise has 

 been very popular and has gone through several editions. He also 

 lectured in the Town Hall at Neath ; in 1851 at the Royal Institution, 

 on the total solar eclipse of that year; and in 1853 on the eclipse of 

 Thales. In 1878 he lectured at Cockermouth on the probable con- 

 dition of the interior of the earth. Besides these there were many 

 other lectures, some of which will be mentioned further on. 



At his Rede Lecture at Cambridge in 1864 Mr. Airy took occasion 

 to point out what appeared to be defects in the system of education 

 as connected with mathematical physics, and he followed up these 

 remarks by a letter to the Vice- Chancellor. To assist in remedying 

 these deficiencies he had already written, in 1861, his now well- 

 known treatise on the theory of Errors of Observation. With 

 the same object he published, in 1866, his " Partial Differential 

 Equations," in which he introduced the novelty of giving stereoscopic 

 views to illustrate the surfaces under consideration. These were 

 followed by a treatise on Sound in 1868. In order to direct the 

 attention of the University to the subject of magnetism, he gave a 

 course of lectures in the Easter term of 1869, at Cambridge. These 

 being attended by crowded audiences, were developed into the treatise 

 on Magnetism which appeared in the following year. 



One of the most remarkable of Mr. Airy's investigations is that 

 in which he determined the mean density of the Earth. He had 

 always been much interested in this subject, and in the fourteenth 

 volume of the Memoirs of the Astronomical Society we find that 

 he assisted Mr. Baily in his important repetition of the Cavendish 

 experiment by contributing the theory and the formulae. In 1826 

 another method had occurred to him which promised to give a more 

 accurate result than either Maskelyne's method of measuring the 

 attraction of a mountain or the Cavendish experiment. His imme- 

 diate object was to compare the force of gravity at the surface and 

 at the bottom of a deep mine, using the pendulum as the means of 

 observation. In the 4 Phil. Trans.' of 1856 he describes the attempts 

 he had made at Dolcoath in 1826 and in 1828, and their failure on 

 both occasions in consequence of accidents, once by fire and once by 

 water. Twenty-six years passed before he was in a position to repeat 

 the experiment for the third time. In 1854, however, a new power 

 was, he tells us, placed in his hands. The galvanic system had been 

 established at the Royal Observatory, and, in the familiarity which 

 he now possessed with telegraphic applications, he perceived that the 

 difficulty of comparing the upper and lower clocks would be entirely 

 removed. The experiment was conducted at the Harton Pit, a mine 

 about 1260 feet deep. The result was that gravity below was greater 

 than that above by l/19286th part, so that the mean density of the 

 earth was 2" 7 times the surface density and 6*6 times that of water. 



