52 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



fixed stars, and then calculated the tabular places for 

 the same instants of time. This last, especially in the 

 case of the moon, was a truly formidable task. The 

 whole labour was methodically performed at the 

 expense of government by a staff of no less than six- 

 teen computers, under Mr Airy's vigilant superin- 

 tendence, and it extended, first and last, over the 

 space of a dozen years. The results, printed in the 

 most compendious form, fill 2200 quarto pages. One 

 consequence of this systematic reduction was, that 

 MM. Leverrier and Adams were separately provided 

 with the places and errors of the Tables of Uranus 

 necessary for their great investigations ; and the 

 Lunar Theory has been enriched with Mr Hansen's 

 latest corrections through the same facilities. The 

 final corrections of the co-efficients of the Lunar 

 Tables are concisely stated by Mr Airy in a paper 

 addressed to the Royal Astronomical Society. 

 (230.) Like Bessel, Mr Airy is thoroughly acquainted 

 The new w ith the theory and practice of mechanics ; and he 

 TransU ^ as a PP^ e ^ his knowledge both to the improvement 

 Circle. of clocks and of proper astronomical instruments. A 

 great and apparently successful experiment of the 

 latter description is now taking place at Greenwich. 

 A meridian circle (combining in effect a mural circle 

 and transit instrument) has been erected, of great size 

 and optical power (6 feet circle and 8J inch object- 

 glass). But the novelty of the apparatus is its great 

 weight and strength. Hitherto artists and astro- 

 nomers have sought to avoid the evils of unsteadi- 

 ness and flexure principally by the elaborate framing 

 and trussing of light materials (chiefly brass tubes), 

 and (in Germany especially) by an elaborate system 

 of counterpoises. The new Greenwich circular in- 

 strument is constructed of as few pieces as possible, 

 of great strength and weight, and in great part of 

 cast iron. The whole was elaborated and prepared 

 for dividing in an engineering workshop, and with 

 the powerful and accurate tools which modern engi- 

 neers use. A man might stand upon the axis with- 

 out producing the smallest bad effect. It is under- 

 stood that the result is perfectly satisfactory as to 

 steadiness and stiffness. Mr Airy has likewise in- 

 vented and erected a new and most ingenious zenith 

 tube (acting by reflection) for ascertaining the place of 

 y Draconis, a star which, at its superior culmination, 

 crosses the meridian of Greenwich almost in the zenith. 

 (231.) There are few projects for the improvement of 

 astronomy in the last twenty years in which Mr Airy 



has not been a counsellor or participator. He has Introduc- 

 at all times given efficient aid to those engaged in the tion . of S al 

 pursuit of science; and has been himself the first to iro-^hod^fMt)' 

 port from America and to establish on a great scale servations 

 the method of recording observations by means of a Greenwtti 

 galvanic pressure produced on a sheet of paper sur- 

 rounding a cylinder ; which cylinder is moved in a per- 

 fectly uniform manner by means of a connection with a 

 clock producing uniform motion on the principle of 

 the conical pendulum. By means of a simple contri- 

 vance the tracer follows a slightly spiral line, so that 

 one barrel may include many hours of continuous ob- 

 servations. He has also established galvanically time 

 signals, and employed them for the determination of 

 longitudes. Besides all this, he has taken part in 

 furthering geodetical measures, in regulating the 

 national standards of length, and in promoting in 

 various ways the collateral sciences. He contributed 

 to the Encyclopaedia Metropolitana a most able 

 analysis of the methods for determining the Figure 

 of the Earth, and his conclusions have been very 

 generally adopted. In the same publication he has 

 given a theory of the Tides, already referred to (78, 

 82). The public and his own university are indebted 

 to him for several very valuable text-books on mathe- 

 matical physics. 



During his incumbency at Greenwich, Magnetical (232.) 

 and Meteorological Observations have been added Ma S ne1 

 to those of Astronomy, and engage the time of four teoroiogi- 

 regular assistants in that department. The astro- cal obser- 

 nomical assistants amount to six, besides occasional vations - 

 computers. It may safely be affirmed that there is 

 not a public establishment in a more complete state 

 of efficiency than is Greenwich Observatory at this 

 moment; and that its traditional reputation, ex- 

 pressed in the emphatic eulogium of Delambre, Art. 

 (150), runs no risk of deterioration under the direction 

 of Mr Airy. 



Our limits will not allow us to refer to the nu- (233.) 

 merous private observatories in England and public ( 

 ones abroad which are contributing useful elements 

 to the promotion of astronomy. It is not too much 

 to say that the activity of Greenwich has set a gene- 

 rally good example. The observations at Oxford 

 and Edinburgh have long been reduced on a syste- 

 matic plan ; those at Cambridge had been already so 

 treated. The establishments where sidereal astro- 

 nomy is particularly cultivated will be noticed in 

 another section. 



servatories 



4. BORDA KATER BAILY. The Figure of the Earth from Pendulum Observations " Reduc- 

 tion to a vacuum;" Mr Stokes. Colonel EVEREST M. STRUVE. Latest Measures of the 

 Earth. M. Foucault's Pendulum Experiment. 



(234.) In the first section of this chapter I have men- 

 Figure of tioned the progress made in determining the figure 

 resumed 1 f the earth ^ measurements of its surface in the 

 early part of this century. I shall now advert 



briefly to the employment of the pendulum for the 

 same purpose. 



Clairaut had published in 1743 a very remarkable 

 theorem which connects the force of gravity in any 



(25 



