Feb. 



j> 



1890] 



NATURE 



345 



modest inquiry — to wit, how far the railways which jar 

 the nerves of Mr. Ruskin so terribly, are desirable neigh- 

 bours for anyone who prefers the earth under his feet to 

 be firm and steady, as it was aforetime, and as it is now 

 sometimes in remote parts of the country on Sundays. 

 We have all noticed, when standing near a passing train, 

 the vibration of the ground under our feet. Though this 

 vibration decreases as we recede from the train, and may 

 at a distance of 50 or 100 yards become insensible to 

 such a coarse test as the actual jarring of our body, we 

 can understand that it may be sufficient to disturb deli- 

 cate instruments at a considerable distance ; and thus 

 affect the use of instruments requiring a steady foundation. 

 Pre-eminent among such are astronomical instruments, 

 and it was very early in the history of railways that 

 astronomers found themselves compelled to fight for the 

 retention of that steadiness of ground in their neighbour- 

 hood which is of vital importance to them, and with 

 which no human agency had previously suggested an 

 interference. It was in 1835 that the question of taking 

 a railway near an Observatory was first raised, in connec- 

 tion with the Royal Observatory, Greenwich ; and an 

 animated discussion resulted in the defeat of the railway 

 company. 



But they have several times since returned to the 

 charge, for Greenwich has always been an atjtractive 

 centre for excursions, and there are many reasons why 

 railway companies find it continually cropping up in their 

 schemes ; indeed, it is only a few months ago that the 

 latest application of the kind was refused by Parliament. 



On June 19, 1835, the Secretary of the Admiralty wrote 

 to the Astronomer- Royal, Mr. Pond, asking for his com- 

 ments on the proposed scheme for a Greenwich-Gravesend 

 railway, passing in a tunnel under a part of Greenwich 

 Park, in which the Royal Observatory is situated. Mr. 

 Pond replied that he had no experience in such matters ; 

 but " the most important observations made at the Royal 

 Observatory are those in which the stars are seen by 

 reflection from a horizontal surface of mercury. It appears 

 to me highly probable, by what I have experienced from 

 sUghter causes, that the passage of heavy carriages, even 

 at the distance of the intended tunnel, might produce 

 sufficient tremor on this surface to destroy the accuracy 

 of these observations." On receiving this reply. Captain 

 Beaufort, then Hydrographer to the Admiralty, wrote to 

 a friend. Commander Denham, asking him to make ex- 

 periments near one of the few existing lines of railroad — 

 that between Liverpool and Manchester— with a sextant 

 and artificial horizon. After explaining the object of the 

 experiments, he says: — ''It would be childish to be 

 guided by opinions and suggestions, when the facts can 

 be distinctly ascertained by means of the Liverpool and 

 Manchester Railroad, and I therefore want you to take 

 your artificial mercury horizon to that railroad, and watch 

 the contact of a star or the sun in altitude with a tele- 

 scope when the train is passing, at two or three different 

 distances, till you come to the outer limit of vibration, or, 

 in other words, to the distance at which the mercury is 

 no longer affected. After you have tried this on the 

 surface, I wish you would then try the same experiment 

 in the neighbourhood of the tunnel, as I presume that 

 the results will be very different." 



Commander Denham's reply is as follows : — " I find 

 the vibration of trains of 120 tons, at a speed of 

 25 miles an hour, affect the mercury as far as 942 feet 

 laterally with the rails, on the same level, and on equal 

 substratum; but vibration perfectly ceases at I no feet, 

 whilst directly over the tunnel no vibration is detectable 

 at 95 feet distance, though quite discernible at 65 feet 

 vertical distance. . . . I am mdebted to the co-operative 

 accommodation of the directors, who allowed trains of 

 extra weight, and at extra speed, to pass down at night 

 hours when the busy hum (of carting carriages and 

 bustle) was completely suspended." 



In the printed report of this correspondence the 

 Hydrographer notes on this letter: "It is proper to 

 remark on the above that Commander Denham's experi- 

 ments depended on observations with a sextant, and that 

 the limits of tremors in the mercury would be far more 

 extensive if viewed by the high magnifying powers used 

 with the mural circle." 



We have quoted this case in detail not only because it 

 was the first experiment of the kind, but because the accu- 

 racy of the results, as interpreted by the Hydrographer's 

 note, has been confirmed by later experiments. This 

 report was adverse to the railway company, who wished 

 to approach within 650 feet of the Observatory ; but they 

 did not relinquish their scheme at once. They suggested 

 various plans — of running trains at slow speeds, or 

 stopping them altogether if the Royal Observatory 

 signalled that an important observation was just going 

 on, and so forth — all of which were open to the objection 

 of looking too well on paper. Meanwhile Mr. Pond had 

 been succeeded by Mr. (afterwards Sir George) Airy, 

 who, in 1836 January, repeated Commander Denham's 

 experiments in the Glebe Meadow, near the Greenwich 

 Railway, but using a small telescope instead of a sextant. 

 He found that "a disturbance in the clearness of the 

 image (in mercury) was perceptible when the train was 

 1 106 feet from the mercury, and the image was almost 

 lost from the violence of the agitation when the train 

 was about 700 feet from the mercury. When the train 

 was 500 feet from the mercury it was impossible to 

 know whether there ought to be any object visible 

 at all." 



The question was ultimately resolved into a decision 

 upon the minimum distance from the Observatory at 

 which a railway could be allowed ; and under strong 

 pressure, Sir George Airy was induced to define this 

 distance as something over 700 feet ; but the position to 

 which the line was thus removed was found to bring it 

 near other buildings, and the project was ultimately 

 shelved. The Astronomer Royal's troubles were, how- 

 ever, only just commencing. In 1840 the London and 

 Chatham Railway Company asked for leave to go through 

 the Park ; being promptly followed by a similar applica- 

 tion from the South-Eastern Company ; and he must 

 needs repeat his experiments and protests. 



His experiments in March 1846 near the Kensal Green 

 tunnel showed that tremor was sensible in the compact 

 clay of Kensal Green to a distance of 1700 feet, but that 

 the tremor was very much diminished where the railway 

 enters a tunnel. Dr. Robinson, of Armagh, made inde- 

 pendent experiments on the Dublin and Kingstown 

 Railway. He mounted a mural circle on an ash post 

 driven deeply into the ground, at a distance of 1655 feet 

 from the nearest point of the line ; and found that the 

 vibration of passing trains gradually shook the instrument 

 away from any position in which it was clamped, so that 

 an object would not remain bisected by the cross wires. 

 His reflection observations were numerous, and he sums 

 them up as follows : '' On these facts it is, I presume, 

 unnecessary to offer any comment, except the simple re- 

 mark that they show clearly that, in a soil such as I have 

 described, a train of no uncommon weight or velocity can 

 produce, at an oblique distance of two miles, such dis- 

 turbance as ought never to be tolerated in an Observatory." 



Sir James South also made experiments, and concludes 

 his report to the Admiralty thus : — " To the observations 

 of right ascension made by reflectioii, the more immediate 

 object of this communication, let me then entreat your 

 Lordships' serious attention, convinced, as I am, that, did 

 they stand alone, they would justify your Lordships in 

 saying \.o present as well as to future x?a\xo2A applicants, 

 ' Within this Park stands the Royal Observatory 

 OF England, and within this Park's walls a 

 Railroad shall never come.'" (The italics and 

 capitals are as in the original.) 



