CLII ROYAL SOCIETY OF CANADA 



SO many principal stations results from the complexity of the tides 

 themselves. 



Tidal differences. — The tides in any region can usually be referred 

 to a principal station by means of time-differences and ratios for height 

 which are constant. Sometimes, although the tide itself is actually later, 

 it is necessary to take the difference from the following tide at the prin- 

 cipal station, owing to reversal of diurnal inequality during the progress 

 of the tide. If constant differences fail to give satisfactory results when 

 applied in either of these ways, some system of variable differences must 

 be resorted to. 



Tliis method of using variable differences in the calculation of local 

 tides from a port of reference, is a matter of great importance. For it 

 serves to reduce the number of principal tidal stations for which har- 

 monic analysis is required, by extending the region that can be referred 

 to each of them. This question is discussed, in relation to tidal data for 

 the world, in a paper by Dr. Dawson entitled " Variation in the Leading 

 Features of the Tide in Different Regions." (Journal, Royal Astrono- 

 mical Society of Canada, July, 1907.) 



Tide Tables. — The data for the calculation of the tide tables for the 

 St. Lawrence and the Miramichi region have been completely revised, in 

 view of the further observations obtained. For the Tide Tables for 

 1910, two additional tables have been prepared: The turn of the current 

 in the Traverse and tide tables for Prince Rupert, B.C. Also, the pocket 

 editions for Quebec and St. John have been extended, and a new table 

 showing the arrival of the bore at Moncton has been added, as this is 

 much desired. These pocket editions which were published chiefly for 

 the convenience of the pilot service, have found much wider use. The 

 edition of the tide tables has now reached a total of 14,000. 



The Great Lakes. — A beginning in observations on the lakes has 

 been made, at the mouth of the Go-home river, on Georgian bay. This 

 work will be supervised by Professor Loudon of Toronto University, 

 some special instruments being provided by this Survey. The record of 

 the water level in Lake Ontario, as recorded at Toronto by the ITarbour 

 Master, is now forwarded regularly to this Survey. 



It is not expected that a tide of any practical importance will be 

 found on the lakes ; but observations of the amount of wind disturbance 

 will be valuable. The wind occasions a wide oscillation or seiche, which 

 is of importance to shipping; as it affects the depth of the water in har- 

 bours during storms, to the extent of several feet in some localities. 

 From the practical side, it is thus chiefly a question of the reduction at 

 times of available depth, involving risk of the grounding of vessels. 



