Tl8 



HARDWICKE'S SCIENCE-GOSSIP. 



To begin then at the beginning ; we must acquaint 

 ourselves with some of the properties of plane and con- 

 cave mirrors. We must enquire what is meant by angles 

 ef incidence and reflection, by the "principal axis," 

 and "principal focus" of the concave mirror, and by 

 " conjugate foci : " must enquire, further, why it is that 

 the ' ' principal focus " is the best, and yet cannot be em- 

 ployed by the microscopist ; and what are the many 

 and strange alterations made in the positions of the 

 secondary foci, when we exchange a " pencil " of 

 parallel rays for one of divergent rays, or of convergent 

 enes ;— or, again, as we merely alter the angle 

 ef incidence of any one of these forms of pencil ; 

 ©r,. as we vary the distance between the lamp and the 

 mirror, or between the lamp and the bull's-eye. 



We may further enquire as to reasons of preference 

 for any particular kind of "pencil," and for any 

 particular angle of incidence ; and must learn how 

 to form accurately such a pencil, how to assign the 

 correct positions of conjugate foci, and how exactly 

 to obtain any selected angles of incidence. Some 

 other hints as to the management of lamp and bull's- 

 eye, and as to a stand for the instrument, together 

 with a few short tables of results, and one of simple 

 rules- and reminders, may be thrown in, as we pass 

 along, or be added by way of conclusion. 



( To be continued.) 



A 



THE RECENT EARTHQUAKES. 



T the recent meeting of the British Association, 

 Mr. W. Topley, F.G.S., supplied some notes 

 on the recent earthquake in the United States, in the 

 compilation of which he had been assisted by a 

 telegraphic despatch from Major Powell, Director 

 of the United States Geological Survey. Mr. 

 Topley observed that during the last ten days 

 there had been earthquake disturbances over a very 

 wide area of the earth's surface. On the night of 

 Friday, August 27, there were shocks all over Greece, 

 and in a smaller degree all over the East Mediter- 

 ranean area. This earthquake wave apparently 

 travelled from east to west. It was first felt in 

 Malta, Calabria, and Naples, and thence it travelled 

 eastwards as far as Alexandria. It did not appear 

 :x) have been felt west of Southern Italy, probably 

 because its westward area of propagation was there 

 beneath the sea. Possibly it was only a coincidence, 

 liut if so a very curious one, that the earliest im- 

 portant earthquake disturbances in the United States 

 should have occurred on Thursday, the 26th, and 

 Friday, the 27th. There had been slight premonitory 

 shocks for two or three days before, but the principal 

 shock was that of Tuesday night, August 31. This 

 had, however, been succeeded by shocks, fortunately 

 of less intensity, which had been felt over a still 

 wider area. As la!e as Sunday ni^ht there were 

 shocks at Charleston. The main shock was felt from 



the southern part of the New England States on the 

 north-east to the western shores of Lake Michigan 

 on the north-west. In the west it was recorded from 

 St. Louis and Memphis on the Mississippi, and 

 from the coast of Florida in the south. Of the later 

 shocks — and these had been very numerous — some 

 were felt in Nevada and some in California. Ex- 

 amining the newspaper telegrams, Mr. Topley said 

 it was probable that there was some error either in 

 observation or in telegraphing as regarded the time 

 at New York, Springfield, and Albany. New York 

 was 420 miles from Raleigh, and the wave was 

 stated to have travelled in three minutes. Albany 

 and Springfield were respectively 330 and 520 miles 

 away, and the wave travelled to each in ten minutes. 

 An important point in Major Powell's despatch, how- 

 ever, was the evidence of the rapid transmission of 

 the main earthquake wave. Major Powell stated 

 that it travelled over the 900,000 square miles at 

 from twenty-five to sixty-five miles per minute ; but 

 by comparing the times given in the telegram with 

 the distances on a map, he (Mr. Topley) found that 

 the velocity varied from 36 to 140 miles per minute. 

 The shock originated in Central North Carolina on 

 Tuesday, August 31, at 9.50 P.M., 75th meridian 

 time, which is five hours in advance of Green- 

 wich time. The time at Greenwich was therefore 

 2.50 A.M. on Wednesday. Although Charleston had 

 experienced the maximum structural damage, this 

 did not appear to have been the exact point of 

 origin. The earliest shock was noted at Raleigh, 

 230 miles to the N.N.E. ; here the time was 9.50. 

 And if they took a scale of intensity, with five at 

 Charleston as a maximum, Raleigh was four ; 

 Memphis (650 miles west), which the wave reached 

 at 9-55> was als0 f° ur 5 Pittsburg (330 miles N.N.W.), 

 which the wave reached at 10 o'clock, was also four. 

 The farthest points at which Major Powell recorded 

 the scale of intensity were Milwaukie, three, at 

 10.6; and Cedar Keys in Florida, two, at 10.5. 

 There did not seem to be any relation between the 

 intensity and time and the surface distance from the 

 area of origin. This last, indeed, they would not 

 have expected to find. They would rather have 

 looked for rapid transmission along certain lines or 

 through certain rock masses. The last important 

 earthquake of the Eastern United States, that of 

 August 10, 1SS4, was carefully investigated by 

 Professor Carvill Lewis. This was found to range 

 along the eastern side of the Appalachian Mountains, 

 nearly along the line where the old earth movements 

 had been greatest. The exact age of the greatest 

 movement there was well known. It was post- 

 carboniferous, for the coal measures had been 

 violently contorted and coal converted into anthracite 

 as the effect of great thrusts from the south-cast. It 

 was pre-triassic, for the trias lay upon the contorted 

 and denuded edges of the older rocks. Put along 

 the same line, and parallel with it, there had been 



