44 



NATURE 



[July 13, 191 1 



LETTERS TO THE EDITOR. 



[The Editor docs not hold himself responsible for opinions 

 expressed by his correspondents. Neithei can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Deformation of Rocks under Tidal Load. 



That a shore-line should be depressed by the weight of 

 a high tide and rise again when the tide retreats is an 

 idea that has occurred to many. Sir George Darwin has 

 on certain assumptions calculated the form and amount of 

 deformation to be expected under given tidal conditions. 

 In the British Association Report for into, p. 40, I showed 



Electrical Discharge — Possible Cause of Flare Spots 

 in Photographs. 



During a recent yachting cruise on the north-easl cuasl 

 I sailed from Holy Island to St. Abbs, arriving off the 



harbour before the fishing fleet came out, and the n 



dropped anchor off the entrance. About 5.30 p.m. s 



ol tin Scotch herring boats sailed out, and I photographed 

 one shortly after leaving the entrance, and another when 

 she was well outside. On having the two negatives 

 developed and printed, I was disappointed to notice that 

 both pictures were considerably marred by a white flare 

 extending from the mast and yard of the sail skywards. 

 At first I put down the flare spot to light leaking into 

 the camera, or some sort of optical halation. On consider- 

 ing the matter more carefully, I was struck with the 

 coincidence of both photographs showing the defect start- 



.-Re 



nd by Tidal Load. 



what had actually been recorded by Mr. W. E. Plummer 

 at the Bidston Observatory, which lies two miles inland 

 from high-water mark. At that place a 10-foot tide re- 

 sulted in an angular deflection of approximately 02". 

 Had the instrument been nearer to the sea a greater 

 change in inclination might have been expected. 



This year, between March 4 and May 24, an instrument 

 similar to that at Bidston was, by the kind permission of 

 the committee of the Royal Victoria Yacht Club, installed 

 in the base of their premises at Ryde. The distance 

 between this installation and high-water mark was 138 

 feet. The displacements due to a 10-foot tide were approxi- 

 mately 0-9". This is more than I should expect to find 

 at Bidston if that observatory were near to the high-water 

 mark. It it would be really greater, then the soft 

 Tertiaries beneath the Solent yield more than the hard 

 sandstones which run seawards from Bidston. This may 

 perhaps be a point of interest to geologists. The astro- 

 nomer with his observatory near a seashore will realise 

 the- extent to which he is handicapped, due to tidal tilt- 

 ing, in relation to the man who makes similar observa- 

 tions twenty miles inland. 



Considering the magnitude of the deflections due to tidal 

 load, the geophysicists may wonder whether we could or 

 could not in a country like Britain obtain satisfactory 

 measurements of a terrain tide due to lunar influences. 

 The definite measurements of the amount of bending which 

 a tide produces on a floor of a shallow dish-like sea bed 

 puts th. hydrographer in a position to calculate the differ- 

 ence between what he observes in the rise and fall "I 

 the tide and what it would be if the bed had been abso- 

 lutely rigid. 



Lastly, a- to the seismologist who has tried to find the 

 relationship between earthquake frequency and tidal load. 

 So far as I know, this has not yet been shown. The 

 reason for this is perhaps because we have not confined 

 our attention to earthquake regions where the effect ol 

 tidal load was mark <i 



Mi. diagram lie: i.l i a half-sized reproduction ol a 

 "graph " at Ryde. The flat crests and sinuses ol the wavi 

 indicate thai the tide lingers foi a considerable time al 



high " and " low. " The reason Foi this is apparently con- 

 nected with the facl thai Ryde is approached by tides from 

 two direction- One enters the Solenl from the easl and 

 hi 1 from the » esl , bul al someu hal diff* 1 1 nl times. 



Shide, rsle of Wight, July 5. Jons Mum 



NO. 2176, VOL. 8/] 



ing from the yard and the mast, and not at all visible on 

 the lower dark sail. I talked the matter over with several 

 friends, and eventually Mr. C. Faraday Proctor jokingly 

 suggested it was evidently caused by electrical discharge. 

 We both took the idea in a sarcastic spirit ; but very soon 

 we realised that the suggestion was not so ridiculous, and 

 was well worthy of serious consideration, particularly as I 

 remembered that the weather conditions had been thundery 

 during the two previous days whilst sailing up from Blyth 



to Holy Island. I then carefully examined other photo- 

 graphs taken on the following day of boats in the harboui 

 and several of these showed evidence of the same effect. 



I enclose herewith three photographs. 



No. 1 (here reproduced) shows the keel boat just sailing 

 out ol the harbour, taken Tuesday, June 20, 5.30 p.m., 

 looking south. 



No. 2 shows boat taken a few minutes later looking 

 east. 



No. 3 shows boats in the harbour looking north, taken 

 about 1 p.m. on Wednesday, June 21. In the skj above 



