May 2, 1892.] 



KNOWLEDGE 



89 



There are some capital photographs ilhistrating the 

 effects produced on the Nagoya Railway Bridge — a lattice 

 girder-bridge. The broken cast iron piers lie on the dry 

 shingly bed of the river, and the original line of the 

 bridge has, in its central part, been deflected up stream. 

 At one place a group of four or five houses has completely 

 sunk into the earth ; only the roofs are to be seen, and on 

 looking under them, instead of the confused heap of 

 rubbish found under other houses, there is merely the 

 surface of the ground, the same level inside as outside ! 

 The unfortunate persons who were in these houses at the 

 time, lie buried, no one knows how deep. 



It might seem at first sight as if it were quite impossible 

 to mitigate the terrible effects of these disturbances, but 

 experience teaches that there are ways in which much 

 destruction to life and property may be avoided. Thus, 

 it is known that on certain sites, buildings are not so 

 much swayed and disturbed as on others only a short 

 distance away. Thus the nature of the ground makes a 

 good deal of difference, and buildings on soft ground, such 

 as is generally found on plains, suffer more than those 

 situated on hard rocky ground. River banks and the edges 

 of cliffs are dangerous sites, because of the forward swing 

 of the free face. The movements at the bottom of a pit, 

 or even in a shallow railway cutting, are less than those 

 upon the natural surface ; hence buildings rising from a pit, 

 or with an area or basement, are less severely shaken. In 

 building in a country much visited by Earthquakes, it is 

 very important to construct with a view to resisting move- 

 ments of a more or less horizontal nature — not merely up 

 and down movements. Consequently, a heavy roof is a 

 thing to be avoided, because by its own inertia it may 

 break the support beneath, and come crashing down. In 

 Italy arches are forbidden in Earthquake regions, for arches 

 fall apart readily when acted upon by horizontal movements. 

 A series of rules have been formulated for Japan, and it is 

 much to be hoped that these rules will receive from 

 builders the attention they deserve. The Nagoya-Gifu 

 Plain is a region where Earthquakes are frequent. Violent 

 disturbances took place in the northern part of Gifu in 

 1826, 1827, and 1859. But there is no doubt that in this 

 district seismic activity is gradually becoming less ; the 

 occurrence of great disasters has been separated by longer 

 and longer intervals. Many interesting problems in 

 seismology have yet to be solved, but Prof. Milne's labours 

 in Japan will at least form a valuable foundation for future 

 workers in this field of research to build upon. 



THE GREAT SUNSPOT AND ITS INFLUENCE. 



By E. W. Maunder, F.R.A.S., 



Assistant superintendinij the Solar and Spectroscopic Depart- 

 ments at the Royal Observatory, Greemeich. 



IT would be a very difficult and toilsome task to obtain 

 a satisfactory record of the perturbations of the 

 magnetic needle, were it not for the assistance which 

 photography has given us. But it is no longer 

 necessary to have relays of observers reading the 

 instruments at intervals of two minutes through every 

 hour of the twenty-four ; by a very simple and effective 

 arrangement the needles record their own positions, not 

 merely at two minute intervals, but continuously. A light 

 mirror is attached to the needle, and a gas lamp or other 

 source of hght is so arranged that a beam of hght pro- 

 ceeding from it and falling on the mirror is reflected off to 

 fall on a drum covered with sensitive paper. The drum is 

 turned by clockwork and revolves once in the twenty-four 

 hours. If then the magnet remains absolutely still, the 



spot of light falling on the paper will leave a blackened 

 trace upon it which, under such circumstances, will form 

 a straight line right round the drum. But if from any 

 cause the magnet should twitch or turn to one side or the 

 other, then the spot of light will be thrown up or down on 

 the drum, and a zigzag will be introduced into the trace. 

 As the light is shut off from the gas-jet by the clock at 

 each hour there is then a slight interruption in the trace, 

 and should a deflection be indicated it is only necessary to 

 see between which hour-marks it lies, and to measure its 

 distance between them to ascertain the exact minute at 

 which the disturbance took place. In order that the hour- 

 marks may be correctly identified, the light is cut off by 

 hand at some specified time. In the copy of the trace for 

 the twenty-four hours extending from noon of February 

 12th to noon of February 13th, given in the adjoining plate, 

 will be seen that this break occurs at about a quarter to 

 nine in the evening of February 12th. 



One result of this automatic method of registration is 

 that it becomes possible to ascertain whether or no a 

 disturbance is truly simultaneous for the magnets at widely 

 separated stations. This is a matter of first importance, 

 for if it be clear that the magnets the world over are all 

 disturbed at practically the same instant, we are precluded 

 from finding the cause in anything restricted or local, and 

 it becomes possible to accept an explanation which connects 

 the disturbance with solar changes. 



The accompanying plates are simple photographic copies, 

 unmanipulated and unaltered, of the original automatic 

 registers of two of the magnets at the Royal Observatory, 

 Greenwich, for the period from noon on February 12th to 

 noon on February 11th. The two magnets in question are 

 those which record the "declination" and the "horizontal 

 force " respectively — the declination, that is to say, the 

 divergence of the needle from the true geographical north ; 

 the " horizontal force "' is a measure of the intensity of the 

 magnetic action, but the total amount of that intensity is 

 not observed directly ; it is observed in two directions at 

 right angles to each other, the one horizontal, the other 

 vertical, the record of the former only being given here. 



From noon on February 12th, for seventeen hours, the 

 trace shows that the magnets were almost quiescent. The 

 bending diO\va of the declination trace during the early 

 afternoon of February 12th, its upward turn towards 

 evening and through the night, until past midnight, are 

 typical of the regular diurnal movement. At times of 

 great stillness this gentle wave motion goes on day after 

 day, unbroken even by the little breaks and ripples which 

 the trace shows in this particular instance. 



And now, before looking at the sudden and sharp 

 disturbance which occurred at about half-past five in the 

 morning of February 18th, let us briefly glance at the 

 reasons already before us, for assigning the sun a dominant 

 influence on terrestrial magnetism. First of all, there is 

 this daily swing, westward in the morning and early 

 afternoon, eastward in the evening and night, a swing 

 which goes on perpetually day after day, which shows so 

 clearly by its conformity to the length of the solar day, 

 and by its being an action, not simultaneous over all the 

 earth, but varying with the local time, that it is con- 

 nected with the position of the sun relative to the place 

 where the observations are being made. Next, there is a 

 yearly, or rather, a seasonal variation, the amount of this 

 daily change and the intensity of the magnetic force being 

 least in the winter — the local winter — rising to a maximum 

 a little before and again a little after midsummer. Here, 

 again, is a second distinct, indisputable local effect, 

 connected with the manner in which the given locality is 

 presented towards the sun. The daily rotation of the 



