Makcii 31, 1910] 



NA rURE 



i^i 



off the bottom, the summit of the ridge was felt with 

 the lead, and the depth over it registered every half- 

 hour from 5.30 a.m. to 8 p.m., August 25, 1840. 



The observations, being carried out with the utmost 

 care under exceptionally favourable conditions of 

 weather, gave an absolutely uniform depth throughout 

 the day, showing conclusively that at that spot there 

 was no rise and fall of tide. The position in which the 

 observations were made was in lat. 52° 27' 30* N., 

 long. 3° 11' 30" E., the moon's age being 27'6 days, 

 and the maximum strength of the tidal stream 16 knots. 



The particular spot for observation, as indicated 

 by Dr. Whewell, was about 30 miles S.S.W. (true) 

 from the above position, but circumstances did not 

 permit of reaching it, and no observations have yet 

 been made there. 



On a former occasion, on July 5, 1838, the moon's 

 age being i3'4 days, at a position about 20 miles 

 S.S.E. (true) from Dr. Whewell's position, using 

 precisely the same' me'thod and under conditions only 

 slightly less favouriible. Captain Hewett found a rise 

 and fall of tide of 65 feet. 



The question of tidal observations in deep water 

 having recently engaged attention at the Hydro- 

 graphic Department of the Admiralty, an apparatus 

 has been devised which obviates to a great extent the 

 difficulties referred to above. 



This apparatus, for use from a ship at anchor, is 

 based on a principle similar to that of the pneumatic 

 self-recording tide gauge now under trial by the Ad- 

 miralty. It consists of india-rubber tubing having a 

 bore of about | inch, supplied in a sufficient number of 

 lengths joined together to allow one end open to the 

 sea to be attached to a weight lowered to the bottom 

 near the anchor. The inboard end of the tubing is 

 attached to the upper part of a closed vertical cylinder 

 4 inches in diameter and about 6 feet high, on the 

 top of which is fitted a small Bourdon gauge of or- 

 dinary pattern. The lower part of the vertical 

 cylinder is in connection with an air-reservoir, and is 

 also connected, by a separate pipe of small diameter, 

 with a large Bourdon gauge of special construction. 



The air-reservoir, charged by a powerful air pump, 

 consists of four cylinders, each of which is similar 

 in size and pattern to the vertical cylinder. The large 

 Bourdon gauge is 12 inches in diameter, very deli- 

 cately made, capable of indicating pressures up to 

 250 lb. on the square inch, and graduated on a reflect- 

 ing surface to obviate the effect of parallax in reading 

 off. It can be accurately read to within i/io lb. 



The method of using the apparatus is as follows : — 

 With the ship lying at anchor, and having sufficient 

 cable veered, the india-rubber tubing should bear no 

 strain. The 12-inch Bourdon gauge being shut off 

 by a needle-valve controlling connection with the 

 remainder of the apparatus, air is pumped into the 

 air-reservoir, flowing from thence to the sea through 



English Channel. — Tidal Observations by Capt. W 

 Date 



1909 



May 18 

 May 19 

 May 2o 

 May 21 



Posuion 



/ 50 22 35 N. 1 

 ( o 35 40 E. ( 

 f SO 9 30 V. 1 

 \ o 49 JsW. / 

 / 50 26 o N. \ 

 1 I 8 oW. t 

 /50 24 10 N. \ 

 ( 2 2 loW. i 



Depth 



Fathoms 



Wind 



Time of 

 High Water 

 h. m. 



Time of preceding 

 Moon's Transit 

 h m. 



tubing and vertical cylinder, controlling connection 

 with the sea, is then closed, and the air reservoir and 

 vertical cylinder charged to a pressure considerably 

 exceeding that of the head of water due to the depth. 

 The compressed air being then admitted to the 12-inch 

 Bourdon gauge by turning the needle-valve, the whole 

 apparatus is again placed in direct communication 

 with the sea by means of the valve for that purpose. 



The air pressure as shown by the 12-inch Bourdon 

 gauge will then steadily fall as the air escapes into 

 the sea, and will continue to do so until the pressure 

 in the apparatus exactly balances that due to the 

 column of water represented by the depth over the 

 submerged end of the india-rubber tubing. When the 

 pointer of the 12-inch gauge ceases to fall and remains 

 quite stationary, the gauge is read off. 



As a column of sea water i foot high, with sec- 

 tional area of i square inch, weighs 0*445 lb-> i* 

 follows that the depth is obtained by the multiplication 

 of that factor by the pressure in lbs. per square inch 

 as indicated by the gauge. The variation in pressure, 

 provided the weight at the submerged end of the india- 

 rubber tubing has not moved its position, is there- 

 fore a measure of the rise and fall of tide. 



Observations with this apparatus have been made 

 successfully in depths of 35 fathoms, and the results, 

 when compared with observations of an ordinary tide- 

 gauge on the beach in the immediate vicinity, were 

 found to agree very closely. For purposes of com- 

 parison, simultaneous observations were taken afloat 

 and ashore at half-hourly intervals during several days. 

 In fine weather an occasional difference of 2 or 3 

 inches might be noted, but it seldom exceeded one 

 inch, or even less. 



On one occasion when observations were being made 

 during bad weather, force of wind 5 to 6, with the 

 ship rolling and pitching considerably, difficulty was 

 experienced in reading the gauge accurately ; the 

 differences observed were consequently somewhat 

 larger, but in no case exceeded 8 inches. The Bour- 

 don gauge used on that occasion has, however, since 

 been vastly improved by the addition of the reflecting 

 surface for the avoidance of parallax, besides other 

 modifications tending towards greater accuracy and 

 facility in reading off. The improved gauge may be 

 expected to give results on which reliance mav con- 

 fidently be placed within a ven,- small margin of error, 

 even under unfavourable conditions. 



The apparatus having been thus satisfactorily 

 tested, the officer commanding H.M. surveying ship 

 Triton was directed to make observations at certain 

 positions in the English Channel, using the improved 

 Bourdon gauge, with the view of verifying the co-tidal 

 lines as drawn by the late Dr. Whewell from theor- 

 etical considerations. 



The results given in the following table are very 

 interesting, and show that the theoretical co-tidal lines 



P. Dawson, R.N., H.M.S. " Triton," May, 1009. 



Lunitidal Time of Range Max. Bar. and Time of H.W. 



Interval Low Water of Tide Current 'Yh'.r. at Dover 

 ft. in. knots h. m. 



h. m. 



h. m. 



24 West 2'3 9 30 a.m. lo 33 p.m. May 17 10 57 4 30 p m. 



Calm 



1030 a.m. II 26 p.m. May 18 n 4 4 30 a.m. 13 9 



E. bS. o-i 10 30 a.m. o 21 a.m. May 20 10 



Lt. air W. 8 50 p.m. 1 46 p.m. May 21 7 



All times are G.M. Time. 



4 15 a.m. 



[ oa.m. 

 May 22 



6 6 



the vertical cylinder and india-rubber tubing. Pump- 

 ing is continued until the small Bourdon gauge ceases 

 to rise, thereby showing that all the water is ex- 

 pelled from the tubing, and that the air is escaping 

 freely from the submerged end, at each stroke of the 

 pump. The valve at the junction of the india-rubber 



NO. 2 log, VOL. 83] 



require considerable modification. It may be hoped 

 that with an apparatus available which enables 

 accurate observations to be carried out without un- 

 necessary loss of time, further information mav 

 eventually be obtained in many parts of the English 

 Channel and North Sea. A. M. F. 



