656 Proceedings of the Royal Irish Academy. 



LXXXVIII. — COMPTJTATIOK OF TiDES — RESULTS OE ThEOEY AND Ob- 



SEKVATioN. By James Pearson, M. A., Ex-Scholar (15th Wrangler) 

 of Trinity College, Cambridge ; Vicar of Fleetwood. ("With Plate 

 XIX.) 



[Read, November 13, 1882.] 



The subject of Tides, upon which, from time to time, I have had the 

 honour of addressing the Royal Irish Academy, is one of which the 

 importance can hardly be overrated, whether we regard it in its con- 

 nexion with physical science or in its reference to practical naviga- 

 tion. A ship, on arriving at her port of destination, requires a safe 

 access and convenient place of discharge, and these cannot be secured 

 unless it is ascertained that a sufficient depth of water will be found 

 to keep her afloat, and that a suitable time has been fixed upon for 

 her entrance into the tidal harbour. Hence the exigencies of the case 

 demand strict attention be paid to the amount of rise and fall of tide. 

 A rough guess is not sufficient ; an error of a few inches may cause the 

 vessel to take the ground, and so to be left high and dry twice in 

 twenty -four hours ; and this for several days, in fact until the return 

 of spring tides supplies water enough for her draught. Now this has 

 a double inconvenience and loss. There is the expense of delay in her 

 discharge, which is often very considerable — wages incurred without 

 work, and time idly squandered. If an attempt is made to lighten her 

 by removing some of her cargo, it may be unsuccessful, and involves 

 expenditure. More than this, the grounding of a ship of magnitude 

 and in full load is most injurious. A severe strain takes place from the 

 efi^ects of which she can hardly ever be recovered ; and if the ground 

 be very hard and uneven, she may "break her back." jN'ow these 

 are not exaggerated dangers ; and therefore whatever can be done to 

 prevent their occurrence is a real boon to mercantile interests. 



Impressed with these considerations, and having all the theoretical 

 information on the subject which a knowledge of mathematics could 

 supply, combined with an ardent love of what I may call an "unfre- 

 quented study," with the singular advantage of having my home within 

 two hundred yards of a self-registering ticle-guage, I have for the last 

 twelve years practically applied myself, not only to the main problem, 

 but also to the discrepancies involved in consequence of atmospheric 

 disturbances, and evidence is now forthcoming to show that the success 

 has been very remarkable. Self-praise is no recommendation, but 

 those who have used the Admiralty Tide Tables for Liverpool during 

 the last five years, have been prompt to testify to the improvement 

 which has taken place in their predictions of the height of tides. The 

 calculations now are based on a modification of Bernouilli's, or the 

 Equilibrium Theory ; and the figures employed Averc, in the first in- 

 stance, taken from Sir John "W. Lubbock's " Elementary Treatise on 



