TRANSACTIONS OF SECTION G. . 795 



ammonia. By adopting direct evaporatiou, the danger is avoided of rendering the 

 ground unfreezable in the event of the escape of the iinfreezable liquid ; the cost of 

 the installation is reduced by dispensing with the unfreezable liquid, and with the 

 apparatus used for rendering it cold ; and the power of the refrigeratiog machine 

 is much better utilised. The process possesses the advantage of being able to freeze 

 the bottom without freezing the upper layers. Thus, when it is necessary to 

 deepen the lined shaft of a mine which has been flooded, the freezing-pipes can be 

 placed inside the lining, without any risk of bursting the lining by the freezing of 

 the water which is inside it. In the case of tunnelling under a river, as the evapora- 

 tion of the ammonia takes place below the water-level, hardly any of the cold is 

 lost in the contact of the pipes with the water ; whereas a great quantity would be 

 lost in employing an unfreezable liquid. 



East Anglian Coal Exploration, Description of Machinery Employed. 



By J. Vivian. 



4. The Effect of Wind, and Atmospheric Pressure on the Tides. 

 By W. H. Wheeler, M.Inst.C.E. 



In this paper it is shown that while a general rule, founded on observations made 

 hy Sir J. W. Lubbock, as to the effect of atmospheric pressure in raising or depress- 

 ing the height of the tides has been formulated, no attempt has yet been made to 

 deduce any law as to the more important eifect of gales of wind ; and shows that 

 the subject is one of considerable iaiportance to navigation, especially to pilots and 

 captains of coasting vessels, who frequently have to cross over bars and shoals in 

 navigable channels with a very narrow margin of water under the keel, while tides 

 are frequently raised or depressed to the extent of several feet by gales. 



In the author's opinion the use of the barometer cannot be made of service in 

 predicting the condition of the tide, as the pressure varies on different parts of the 

 coast, and in order to calculate its eflect on the tide the direction of the gradient 

 of pressure and the locality of high and low pressure must first be known. This 

 can only be ascertained by consulting the weather chai'ts issued from the meteoro- 

 logical office. This source of information is not available on board ship or at many 

 of the smaller ports. A rapid alteration in the pressure of the atmosphere is 

 almost always accompanied by wind, which affords a more readj' and reliable 

 guide for the immediate purposes of navigation. 



From an analysis of two years' tides at the Port of Boston, and excluding 

 occasions when the element of wind would affect the case, the author found that 

 out of 152 observations, 61 gave an opposite result to that which would have been 

 expected ; a high barometer frequently being accompanied by a high tide, and a 

 low barometer by a low tide. 



On the other hand, with few exceptions, it is found by experience that when 

 the wind blows with any force along a coast in the same direction as the main 

 stream of the flood tide, the tides at all the ports along that coast will be higher 

 than the calculated height given in the tide tables ; and when the wind blows 

 against the flood tide, high water will be lower than calculated. 



The author gives numerous instances of the effect of gales in raising and lower- 

 ing the natural height of the tides, and tables showing the effect of the gales of 

 November 1893 and 1804 on the tides at the principal ports round the coast of 

 Great Britain. These figures show that the variation is on some occasions as 

 much as from 5 to 6 feet, and the difference in the height between two succeeding 

 tides as much as 8 feet. 



From an analysis of the register of tides at Boston Dock on the East coast 

 over two years, the author found that 24 per cent, of the whole tides recorded 

 were sufficiently affected by the wind as to vary 6 inches from the calculated 

 height ; thirty varied by 2 feet, seven by 3 feet, six by 3i feet, three by 4 feet, two 

 by 4i feet, one by over 5 feet, and one by 6 feet 3 inches. 



