There is a further reason for this continual observation, not only in the case of the sun, which is our great standard star, since from it we derive the right ascensions of the stars, and it is also our great timekeeper; not only in that of the moon, but also in the case of the planets. Their places as computed need continually to be compared with their places as observed, and the discordances, if any, inquired into. The great triumph which resulted to science from following this course--to pure science, since Uranus is too faint a planet to be any help to the sailor in navigation--is well known. The observed movements of Uranus proved not to be in accord with computation, and from the discordances between calculation and observation Adams and Leverrier were able to predicate the existence of a hitherto unseen planet beyond-- 'To see it, as Columbus saw America from Spain. Its movements were felt by them trembling along the far-reaching line of their analysis, with a certainty hardly inferior to that of ocular demonstration.'[5] [5] From Sir John Herschel's address to the British Association, September 10, 1846, thirteen days before Galle's first observation of the planet. The discovery of Neptune was not made at Greenwich, and Airy has been often and bitterly attacked because he did not start on the search for the predicted planet the moment Adams addressed his first communication to him, and so allowed the French astronomer to engross so much of the honour of the exploit. The controversy has been argued over and over again, and we may be content to leave it alone here. There is one point, however, which is hardly ever mentioned, which must have had much effect in determining Airy's conduct. In 1845, the year in which Adams sent his provisional elements of the unseen disturbing planet to Airy, the largest telescope available for the search at Greenwich was an equatorial of only six and three-quarter inches aperture, provided with small and insufficient circles for determining positions, and housed in a very small and inconvenient dome; whilst at Cambridge, within a mile or so of Adams' own college, was the 'Northumberland' equatorial, of nearly twelve inches aperture, under the charge of the University Professor of Astronomy, Professor Challis, and which was then much the largest, best mounted and housed equatorial in the entire country. The 'Northumberland' had been begun from Airy's designs and under his own superintendence, when he was Professor of Astronomy at Cambridge. Naturally, then, knowing how much superior the Cambridge telescope was to any which he had under his care, he thought the search should be made with it. He had no reason to believe that his own instrument was competent for the work. [Illustration: THE NEW OBSERVATORY AS SEEN FROM FLAMSTEED'S OBSERVATORY.]