XII ROYAL SOCIETY OF CANADA 



We do not refer particularly to the practical endeavour of the de- 

 partment to make the results immediately known to the shipping com- 

 munity, and in giving special attention to the currents met with on the 

 leading steamship routes. We select rather the points which add to our 

 knowledge of the tides themselves and the phj'sical characteristics of the 

 Gulf of St. Lawrence as ascertained by this surve}-. 



Tides and Tide Tables. 



In the tidal branch of the survey a new departure has been made 

 this year in supplying tide tables for Halifax and Quebec to the leading 

 almanacs for publication. This is the first time that tables derived from 

 direct observation of the tides have been av'ailable for any Canadian 

 ports. 



Up to the present time, such tables as were published have been 

 based upon fixed ditterences from distant ports, usually on the other 

 side of the Atlantic, and the time of high water has consequently been 

 .seriously in error during the course of the lunar month, more especially 

 at the quadratures. Also, the time of high water only has been given, 

 whereas the new tables give the time of low Avater also, and the height 

 of the tide at both high and low water above a carefully determined 

 datum plane. This is valuable with reference to the depth of water 

 available in the tidal portion of the St. Lawrence ship channel, and also 

 for vessels entering the dry docks at Halifax and Quebec. Where it has 

 been possible to do so, the datum plane adopted is made co-incident with 

 the level of low water at spring tides, as used for the Admiralty charts. 

 There is much discordance in the matter of datum or reference planes in 

 our cities; as city levels, railway levels, and levels for the construction of 

 puljlic works, are frequentl}' referred to different planes of reference, and 

 these often do not correspond with the Admiralty datum. This is some- 

 times complicated further by the absence of good reference marks from 

 which these levels can be c(n-rectly ascertained. The care that is now 

 being taken in reducing the tidal observations to a fixed reference level, 

 will unable them to be utilized for the correct determination of reference 

 planes at low water and mean sea level for our principal ports. This is 

 not only of direct value to shipping, but it also affords a good reference 

 plane for harbour works, city sewage works, etc. In some cases, also, 

 property is defined by the low water line. 



The time used in the tide tables is Standard time in all cases, count- 

 ing from Oh. to 24h. from midnight to midnight. The tidal differences 

 lor other places which accompany the tables are also computed to give 

 the time of high and low water in Standard time. This has the advan- 

 tage of enabling a captain to read oft' the tide on his own chronometer, 

 by allowing an even number of hours from Greenwich mean time, without 

 the trouble of reducing for longitude and computing local time. It is 



