May 29, 1884] 



NA TURE 



altered his opinion since he presented his paper on March 19, 

 1883, to the Academy of Sciences ; but, as I have shown, hedut 

 make the statement lie now repudiates; and even if the result of 

 my calculations, founded upon that statement, do not agree with 

 what he now considers to be necessary, he has no right to attri- 

 bute the discrepancy to any error of mine. 



M. Lcewy then proceeds to show that it is possible to attach a 

 weight of three-quarters of a ton to end of cross tube. This, I 

 need hardly say, I never disputed; what I did say was, "The 

 absurdity of hanging this three-quarters of a ton" (i.e. weight of 

 mirror, objective, and all their supports and attachments) "on 

 end of cross tube, am i yet calling the instrument cue of precision, 

 is too apparent to need demonstration." If I were asked if it 

 would be possible to hang three-quarters of a ton on each end of 

 the Greenwich transit, I might be able to reply in the affirma- 

 tive, but if I were asked to guarantee that the instrument would, 

 under its new conditions, be as perfect an instrument of precision 

 as it is in its present state, I would not be inclined to risk my 

 "reputation " by any such guarantee, and yet M. Lcewy compares 

 the equatorial coudi To a transit instrument in stability. 



Lastly, on the question of expense. M. Lcewy is anxious to 

 know where I obtained my information, but as I am content to 

 accept his own figures (see his letter in your issue of May 1), so 

 far as the equatorial coudi is concerned, there is no occasion to 

 discuss this point. I take the 12-inch size, as it is the nearest 

 to the only one completed, and most likely to be accurate. A 

 12-inch equatorial coude is estimated at 44,000 franc-, i.e. say 

 1 760/. 



Now, in estimating the relative costs of the two forms, your 

 readers will agree with me that for our purposes the fair com- 

 parison is between the equatorial coudi and such equatorials as 

 are most generally in use in this country, and it is well known 

 that for 1760/. a first-rate 12-inch equatorial and dome can be 

 procured, and this is as nearly as possible what I said, and I 

 have to thank M. Lcewy for furnishing me with materials to 

 prove my case with so little trouble. 



My paper at the Royal Dublin Society (so far as concerned 

 the equatorial coude) was mainly confined to showing that in 

 consequence of its complication it presented many difficulties in 

 its manufacture, particularly for large sizes, and I considered (as 

 I do still consider) that too much w-as sacrificed in endeavouring 

 to make it an instrument of precision, and to obtain universality. 

 The very fact of its being difficult to construct renders any 

 success the more creditable, and I gladly take this opportunity 

 of expressing my admiration for the excellence of the optical 

 work of the Messrs. I lenri, which appears to have withstood the 

 enormous strain put upon it by the peculiarity of construction of 

 the instrument. I still consider, and in this opinion I am joined 

 by the several astronomers of eminence to whom I have spoken 

 on the subject, that the good results are due to the excel- 

 lence of the optical work, and have been obtained in spite of, 

 and not by reason of, the peculiar form of the instrument. 



And now I would say a few words generally on the comparison 

 which has been instituted between the two forms. The claims 

 of the instrument which I propose are very modest. 1 simply 

 claim for it that by its peculiar construction I can obtain an in- 

 strument of large aperture at about one-fourth the usual cost, and 

 that the observer can be situated in a most comfortable position, 

 and free from all the various inconveniences of ordinary observ- 

 ing. I do not claim that the instrument will be one of precision, 

 or that the images will be better after reflection from the mirrors, 

 or that it will be universal, or that it will do all and everything 

 which the equatorial coudi will do at four times the cost. What 

 the equatorial coude claims your readers already know. Like 

 many other matters, this also will probably resolve itself into 

 one of cost. If a director of an observatory has 1760/. at his dis- 

 posal, it is for him to decide whether he will have a 12-inch 

 equatorial coude, which commands the whole visible heavens, 

 or a 24-inch telescope on my plan, sacrificing in this case about 

 20 near the Pole ; or, putting it another way, he may consider 

 the question whether he will spend 1760/. on a 12-inch equa- 

 torial coudi ox 500/. on one of my form of same aperture. 



On this matter I shall have more to say in my second letter, 

 in which also I propose to answer all the various objections M. 

 Lcewy has raised to my form. It may, however, be interesting 

 for him to learn that, with a single unimportant exception, he has 

 not raised an objection which has not already been discussed 

 and provided for in the new instrument ; but he has suggested 

 lo me another objection to the equatorial coudi which I shall 

 also treat of in that letter. HOWARD GRUBB 



Rathmines, Dublin, May 19 



The Earthquake 



My yacht, the Glimpse, lay on the ground in the River Colne 

 at East Donyland, about half a mile above Wivenhoe, and as 

 soon as I was able I joined her in order to study the effects of 

 the late earthquake. I remained in the district about a fort- 

 night, and examined the greater part of the focus of disturbance, 

 over an area of about eight miles long by six broad. I distin- 

 guished on the ordnance map by appropriate marks (1) those 

 places where the shock had been so violent that not only nearly 

 all the chimneys had been knocked down but a large proportion 

 of the house walls cracked and some boundary walls thrown 

 down ; (2) those where it had been less violent, many of the 

 chimneys having been thrown down, but few or no houses 

 cracked ; and (3) those where it had been only sufficiently vio- 

 lent to throw down a few isolated chimneys. This third district 

 extends in some directions much beyond the part examined. 

 District No. 2 may be said to trend from Wivenhoe south-west 

 to somewhat south of Little Wigborough, but sends .1 small, 

 narrow branch north-west up the Colne valley to Colchester, 

 The main part of District No. 1 is at Peldon, Langenhoe, and 

 Strood Mill, but there are two well-marke'd outliers, one at 

 Wivenhoe and another at Mortimer in Mersea Island. At and 

 near Wivenhoe the intensity of the shock seems to have 1 >cen 

 greatest at low levels, and such a supposition would explain the 

 character of that outlier, but no such explanation is applicable 

 to the outlier at Mortimer, since the chief damage there is at 

 a high level, and I was unable to discover any reason for its local 

 character. 



A great part of my attention was directed to such facts as 

 indicated the direction in which the disturbance moved. The 

 mate of the Glimpse was on deck, and say- that the yacht was 

 first, as it were, moved violently forwards to the west, and then 

 even more violently backwards to the east. All the circum- 

 stances of the case make this a very good observation. In trying 

 to determine the direction of the shock from the effects, I have 

 taken great care to select such cases as would mark the first 

 shock, and not the recoil. Unless this be done, no true result 

 could be obtained, since very commonly the chimney- at one 

 end of a house have been thrown down by the direct shock and 

 those at the other end in the opposite direction by the recoil. On 

 the whole I was able to observe nineteen cases which I looked 

 upon as satisfactory. Almost all these vary from east to south. 

 Perhaps the shock was rather more from the east at Wivenhoe 

 than at Peldon. The mean of the whole is very nearly true 

 south-east, which may be said to agree with the axis of chief dis- 

 turbance as laid down by me on the map. The only case which 

 is doubtful is that of the church at East Mersea. The manner in 

 which two portions of the tower have been thrown down seem to 

 indicate a shock from north-west. If this could be relied on 

 with perfect confidence, it would show that the church lay on the 

 south-east side of the vertical line, but I saw nothing else to con- 

 firm such a conclusion, and I think it quite as probable that the 

 damage was done by the recoil which over the greater partol the 

 district was from that same north-west direction. If this supposi- 

 tion is correct, the shock came up from below somewhat obliquely 

 from south-east under East Mersea, where scarcely any damage 

 has been done, and was most violent along the stroke of the 

 wave at a distance of about three miles to the north-west. This 

 and the general character of the area of chief disturbance seems 

 to me to point to some very irregular distribution of hard rocks 

 at a considerable distance below the surface. H. C. SORBY 



Yacht Glimpse, Queenborough, May 25 



In your issue of the 8th inst. (p. 31) Dr. J. E. Taylor draw 

 attention to the fact that sound preceded the Langenhoe earth- 

 quake for an appreciable period of time. A similar phenomenon 

 has often been recorded, but as I cannot just now quote another 

 instance, allow me to put forward a personal one. 



On the morning of Monday, July 11, 1853, I had just gone to 

 bed when I heard a heavy fast-approaching rumbling sound 

 coming from the direction N.W. \ N. I was in St. Jean'de Luz. 

 and had stopped at an inn which skirts the high road from 

 Bayonne to Madrid. The noise was coming nearer with the 

 speed of an express train, and knowing that the only heavy coach 

 which plied in those days could no p h an hour, I con- 

 cluded that an earthquake was coming and got up to look at my 

 watch, which I had left on a table at the opposite corner of the 

 room ; it was 20m. Ss. past midnight. When the noise seemed to 

 issue from the ground under me, the whole house shook ; it 

 was then 24m. 8s. past midnight. Although occurring at a time 



