124 



NA TURE 



I /itnc 5, i! 



No doubt universality unaccompanied by such disadvantages 

 as the equatorial coude" possesses is very much to be desired, but 

 t do not yet see how it is to be obtained. With respect to the 

 solitary original objection that M. Loewy lias raised, viz. that the 

 light reflected by the mirror varies with the different angles of 

 inclination, and therefore renders the instrument unfit for photo- 

 metric researches, I confess this objection did not occur to me 

 before, and I am inclined to think M. Lcewy is right, and that 

 my instrument will not be well adapted for photometric re- 

 searches, but I ask: Is the equatorial coude any better? On 

 consideration it will be evident to your scientific readers that the 

 light after the first reflection is elliptically polarised, and if so 

 the quantity reflected by the second mirror is variable at the 

 various angles of declination ; consequently photometric obser- 

 vations made with the equatorial coudk cannot be relied upon. 



In attempting to prove his case M. Lcewy gives in his first letter 

 a considerable number of numerical details, and no doubt most of 

 your readers have taken these figure as correct. I will ask them, 

 however, to verify for themselves a few of them, and the result 

 will, I think, show how very loosely M. Lnewy has put these data 

 together. For instance, he mentions the weight of a 40-inch 

 mirror, whose thickness is just one-sixth of the diameter, to be 

 380 kilos., and he calculates (see further down) that a mirror of 

 38 inches diameter and proportional thickness would weigh 280 

 kilos. — 100 kilos, less. Now, if the thickness be proportional, 

 the weight should be as the cube of the diameter. If your 

 readers will try this themselves, they will find that M. Lcewy has 

 in this case exaggerated the difference to the amount of about 

 100 per cent. 



I find I omitted to notice just one point in Mr. Lcewy 's first 

 letter. He says : — " If Mr. Grubb had looked at the drawing 

 which I published in the Journal de Physique of last year, he 

 would have seen that it is almost identical with that which he 

 has communicated to the Royal Dublin Society, so far as the 

 general arrangements for sheltering the observer and instrument 

 are concerned." 



Permit me to inform M. Lcewy that this would hardly have 

 been a novelty to me last year, inasmuch as I had such arrange- 

 ments not only on paper but in actual work for some years 

 back, and a description of the same was published in the Royal 

 Dublin Society's Proceedings, April 1879. 



Your readers will see from the foregoing that M. Loewy's 

 whole letter is based ort a series of misconceptions of statements 

 in my paper in the Transactions of the Royal Dublin Society. 



Some of the mistakes that M. Lcewy has fallen into were per- 

 haps due to the fact that the plate issued with the Royal Dublin 

 Society's Transactions was merely a diagram without details, 

 introduced to illustrate the principle of the mounting. He 

 assumes that details not figured in the diagram are not to be 

 provided, in spite of the fact that in the text of my paper I 

 discussed several of them. 



M. Lcewy occupies nearly half a column of Nature in speak- 

 ing of the labour involved in working this instrument, because 

 no tube is shown in the diagram connecting the equatorial part 

 with the ocular ; all this trouble would have been saved if he had 

 read my paper a little more carefully, for then he would have 

 found that not only did I say, "In most cases it would be 

 desirable to have a connecting tube.'' but I even discuss the best 

 form of tube for the purpose. There are some special cases in 

 which a tube would not be actually necessary. 



It appears to me that M. Lcewy is very unnecessarily dis- 

 turbed in his mind by the advent of my instrument. No doubt 

 the equatorial coude and my siderostatic telescope have each their 

 own sphere of work, and there may be room for both. An 

 observatory having at its back a generous individual who (as was 

 stated at a late meeting of the Royal Astronomical Society), has 

 already expended a quarter of a million on astronomical ob- 

 servatories anrl is willing to spend more, can afford a large 

 instrument perhaps on M. Loewy's plan ; but as all observa- 

 tories are not equally fortunate, there may occasionally be one 

 found which will be glad to get equally great optical power 

 at one-third the cost. - 



I barely alluded in my last letter to this question of cost. On 

 this point it may be desirable to supplement what I have said in 

 my former letter, bearing in mind that the cost of the instrument 

 will, as I have above stated, depend somewhat on whether or 

 not the objective is achromatised on the ordinary principle or 

 that of the dialyte. 



In order to put the matter of cost in the clearest light, let us 

 consider the four forms which we have at present to select from, 

 viz. the ordinary equatorial, M. Loewy's equatorial coude, my 



siderostatic telescope with objective achromatised in the ordinary 

 way, and the same instrument with objective achromatised on 

 tile dialyte principle. 



Let us consider first what apertures we can obtain in the several 

 forms for a given sum ; assuming M. Lcewy's figures for the 

 equatorial coude. For 1760/. can be obtained — 



(a) Equatorial coude of 12-inch aperture. 



(b) Ordinary equatorial of 12-inch aperture, including its dome 

 and observatory. 



(c) Siderostatic telescope with objective achromatised in the 

 ordinary way of 18 inches aperture. 



(d) Siderostatic telescope with objective achromatised on the 

 dialyte principle of 24 inches aperture. 



It would be for the astronomer to say whether the double 

 aperture of the objective would not more than counterbalance the 

 disadvantages of want of absolute universality. 



Let us, secondly, consider for what prices the same aperture 

 could be obtained in the various forms : — 



(a) Equatorial coudi 12" aperture _ ... ... _£i76° 



(b) Ordinary equatorial of 12" aperture, including 



dome and observatory 1760 



(c) Siderostatic telescope of 12" aperture with ob- 



jective achromatised in the ordinary way ... IOOO 



(d) Siderostatic telescope of 12" aperture, with 



objective achromatised on the dialyte principle 500 



The difference between cost of equatorial coude and sidero- 

 static dialyte (about 1200/. for this size) will probably be con- 

 sidered by the purchaser rather too large a sum to pay for the 

 possibility of examining the 6 per cent, of the northern hemisphere 

 which is beyond the reach of my siderostatic telescope, parti- 

 cularly when it is borne in mind that that portion is the least 

 important part of the heavens. 



M. Lcewy does not say whether the 1760/. includes cost of 

 observing hut. If not, the comparison is still more striking, for, 

 although the equatorial coude requires a special building, my 

 siderostatic telescope does not. Howard Grubb 



Dublin, May 27 



The Earthquake 

 Chan'CE brought me to Colchester about a week after the 

 earthquake, and since then I have been amusing myself mapping 

 the effects of it, and hope to read a paper on the subject at the 

 meeting of the Royal Geological Society, Ireland, next month. 

 In the meantime I would like to draw attention to a few of the 

 general facts that seem not to be recorded. 



The area of structural damage lies at and southward of Col- 

 chester, principally to the west of the Colne estuary, and in it 

 there are five smaller areas in which are found the greatest 

 damage. These areas occur in the following order: — Wiven- 

 hoe, Peldon, Abberton and Langenhoe, Colchester, and West 

 Mersea ; each of these have two or more well-marked margins ; 

 where these margins can be easily studied, there are found to be 

 lines of breaks, and alongside one, or in places two of them, the 

 greatest destruction occurred, while at the other side of such 

 lines the damage is a bagatelle in comparison. 



Thus at Wivenhoe, where there was the greatest damage done, 

 the shock came from the north-east ; but when it reached the 

 break of the Colne River valley, it seems to have recoiled as if 

 from a percussion blow. Westward of the estuary of the Colne 

 the damage at Rowhedge was slight when compared with that 

 at Wivenhoe, while at Hornwood it was still slighter, although 

 the last is only divided from Wivenhoe and Rowhedge by the 

 valleys of the Colne and Roman Rivers. 



At Peldon, where the shock appears to have been nearly as 

 bad as at Wivenhoe, the damaged area is very well defined, 

 being bounded northward and southward by stream valleys. 

 The shock seems to have travelled southward and to have re- 

 coiled from the southern boundary, causing excessive damage 

 alongside it. Here also the shock appears to have had a rotary 

 motion, which possibly may be due to the recoil against the 

 southern boundary. 



In the Abberton and Langenhoe area the shocks seem also to 

 have had a rotary motion, the main direction seems to have 

 been from the south-east ; here the greatest damage occurs at the 

 western boundary. 



At Colchester the shock was going north, while at West Mersea 

 it went south. In both of these places the boundaries of the 

 areas are in part obscure. In the first, however, we can trace 

 the tract of maximum damage from Head Gate along the south 

 Roman wall and eastward to Colne valley, east of which very 



