That these results may be more easily appreciated, I have condensed the most important of them into the preceding Table, which gives in one view the distance at which that amount of disturbance begins and ends which produces the cross, that at which the far greater one occurs causing the appearance (5) (a system of three or more parallel rows) wherever it does appear, and the velocities and weights of the trains when known.

It is evident from this Table that the tremor which is sufficient to produce that disturbance of the mercury which shows a cross of stars is propagated to considerable distances-in one case to 1176 yards; and 24 per cent. of the entire are above 1000. Such distances do not pass the northern end of the tunnel, but go far beyond the southern. In the latter case the vibrations are excited while the train is in an open cutting; and those who suppose that the tunnel has much power in deadening them would of course expect that they would be sensible at a greater distance than at the other end. This does not seem, however, to be the case: and the Table shows that in this respect there is very little difference, if we take into account another cause of inequality, namely, that the tremor is manifested > further at the exit than at the entrance of the train. The column headed "Exit" shows by s. that the exit was at the South end, and the entrance at the North.

Now, when the observations are examined where both were noted, we find that the limit of the cross is greater at the exit than at the entrance in 29 out of 39, or 74 per cent. of s., and 12 out of 16, or 75 per cent. of N. The reason of this, I suppose, is that the long-continued action of the train on the rails tends to produce a greater and more prolonged undulation in the mercury.

But the equal percentage shows that there is really no protecting power in the tunnel against the lateral propagation of tremors, whatever may be the case immediately above the crown.

In general one might expect trains to produce disturbance in proportion to their speed and their weight. To a certain degree this is true; but the exceptions are sufficient to show that other influences must be taken into consideration. Examples of high speed with comparatively small effect are afforded by the observations on March 18, II.; 19, III.; and 29, V. Others of the reverse conditions are given by February 24, I., II.; March 18, VI.; and specially March 16, III., in which with a velocity of only 11.4 miles the cross was shown at 1110 yards. This it may be remarked is a decisive proof that any plan of protecting an observatory by slackening the speed of trains passing near it is entirely useless, even if it could be enforced.

It is probable that one cause of this high disturbing-power in slow trains is that already referred to, the long-continued accumulation of vibration, the quick ones passing beyond distance before the mercury has got into full vibration, the others having full time to do their work though with less. intrinsic force. On this account also long trains are more disturbing than short.

The engine is not so paramount a disturber as might be expected,-the heaviest, and even a pair of them, not causing more tremor than occurs with the smaller ones.

In taking the cross of stars as the test of disturbance, I must observe that I do so, not because it is the earliest which appears, but because it marks distinctly an agitation greater than what is likely to occur at an observatory subject to ordinary perturbations. These produce in such a mercuryvessel as I used a single line of stars perpendicular to the length of the vessel. It should seem that then only one set of undulations fit to produce these images is excited in the mercury, the direction of which is regulated by the sides of the vessel *. The existence of the cross shows that a second set of waves perpendicular to the first has been developed : this always happens if the sides of the vessel are equal; and its occurring when they are so unequal as in the present case seems to indicate a corresponding excess of the power which causes them. If the agitation be still greater, it seems as if each of the images which form the cross became the origin of a row of secondary images, the result of which is the form (5), a series of parallel rows of stars varying from two to ten, or even filling the whole field. This token of ultra disturbance is confined between lines making angles of 45° with the perpendicular to the rails-in other words, to distances under 427 yards, and when the train is nearly in the centre of the tunnel. It is (except in two instances) only seen when the cross is visible beyond 1000 yards: when the agitation is still further increased the images vibrate in every direction, and with yet more of it the whole becomes a mass of nebulous light; of both which some examples may be found in these observations.

The opinion maintained by the late Mr. Robert Stephenson, that much of these railway tremors were due to the sound of the train, although not probable, induced me to try some experiments by firing cannon, maroons, and rockets at various distances.

One of these cannons (for I had two, each of a pound calibre) heavily loaded, at 300 yards produced (5), cross, and line simultaneously with my hearing the reports; but all disturbance was over in about 1.5 second. At 2020 yards there was the cross synchronous with the report, and of the same momentary character; and even at 3000 yards the cross could be traced. This seems to have been due to the momentary impulse of the sound-wave, for the continuous roar of two-pound rockets fired at 82 feet from the mercury, though very loud, disturbed it very little; while the explosions of eight ounces of powder in their heads about 800 yards above the ground produced all, the (5), cross, and line. A still more interesting experiment was, firing the cannon in the tunnel at the point where the perpendicular from the observatory met it. In this case two disturbances were seen-one propagated through the ground, the other through the air with about a second of time interval. The sound probably made its way chiefly * For details on this I may refer to my Report to the Admiralty, published by order of the House of Commons, July 6, 1846.

through the shafts; but even had they been closed, it seems unquestionable that the report, and of course the sound of a train, would travel through the earth *.

I should have prosecuted these researches further, especially in reference to the velocity with which these tremors are propagated through the ground, but that Lord Auckland's letter to me led me to hope that all danger to the Royal Observatory was past, never to return. I therefore contented myself with reducing the observations I had made. As, however, the Railway Moloch seems never likely to be satiated with victims, and as the observatories of Oxford, Armagh, and again that of Greenwich have been marked for sacrifice, it seems to me a duty to place before the public the facts which had been collected at a great expense of labour, and some pecuniary outlay.

They were made without any bias, or any motive but a desire to ascertain the actual truth; and in addition to their bearing on practical astronomy, I hope that they may not be without use in reference to some other departments of science.

January 7, 1864.

Dr. W. A. MILLER, Treasurer and Vice-President, in the Chair.

The following communications were read:

I. "Extract of a letter to General Sabine from Dr. OTTO TORELL, dated from Copenhagen, Dec. 12, 1863." Received December 18, 1863.

The Swedish Diet has given the necessary money to complete the survey for the measurement of an Arc of the Meridian at Spitzbergen.

When the proposal was submitted to the Diet by our Government, at the instance of the Academy of Sciences at Stockholm, it was passed without opposition in the three first houses of the Diet (viz. the Nobles, the Clergy, and the Burghers). In the fourth house (the Peasants), only one Member opposed the proposal, on the ground of the high amount of the Budget. He was replied to by seven or eight other Members, advising that the house should not oppose a grant which had for its object to advance science. In

* An interesting fact was observed with the maroons. They were fired vertically from a mortar twenty feet from the observatory, and had fuses which gave them flight for six seconds. The mercury showed the usual intense disturbance when the mortar was fired, and also at the explosion of the maroons in the air. But there was also an intermediate disturbance which I cannot explain but by supposing it to be as it were an echo of the earth-wave caused by the discharge of the mortar and reflected from the masonry of the tunnel. I showed it to the Marquis of Blandford, to Lord Alfred Churchill, and to Professor James McCullagh; unfortunately the nights Dr. Robinson and Mr. Warburton accompanied me to Watford, not a single star was visible. On repeating the experiments at Campden Hill, nothing of the sort occurred.

VOL. XIII.

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