182 Mr. Longmire on the [March, 



N. B. As my house, or rather the site of my barometer, is 

 situated 105*9 feet above the sea level, it must be necessary to 

 add 0*104 inch to all the barometrical heights in the foregoing 

 tables for the correct heights of mercury. The tables contain 

 the means of three daily observations; viz. 8 p.m.; 1 a.m.; 

 and 10 a. m. The most prevailing wind of the 24 hours is only 

 given ; and the register thermometer placed in the sun is insu- 

 lated six feet from any thing capable of reflecting heat, and the 

 scale of it is marked on the glass tube itself. 



Hehton, Jan. 4, 1S23. M. P. MoYLE. 



Article IV. 



Essays on the Construction of Sea Harbours. 

 By Mr. J. B. Longmire. 



(To the Editor of the Annals of Philosophy .) 



SIR, Whitehaven, Jan. 9, 1823. 



This subject being new to the public, you may deem the 

 following essays worthy of insertion in the Annals, more espe- 

 cially as you occasionally have papers in it on civil engineering. 



The matter under consideration naturally falls under two 

 parts ; namely, entrances into sea harbours ; and the situations 

 and relative positions of the piers of such harbours. 



I. Of Entrances. 



All entrances into sea harbours may be classed under two 

 heads : in the first, they face the sea, and admit the surf;* and 

 in the second, they face the calmest quarter, while the surf 

 passes to the lee shore without entering the harbour. 



In constructing a harbour to make a proper entrance is a very 

 important task ; as a harbour that is safe within loses much of its 

 value, if not accessible in every wind that a ship can approach it 

 from the main ocean. 



Before describing the theory of entrances, it is necessary to 

 show how vessels approach a harbour in different winds. A ship 

 can sail best with the wind, and her course to any object to the 

 leeward is in a straight line ; and although she cannot sail 

 directly against the wind, she can either reach or pass any 

 object to the windward, by making alternate approaches, which 

 have angles of 6^ points, or 73° 7' 30", with the wind : this is 

 called tacking. When a vessel is sailing with the wind from the 



* The surf is a term applied to express the state of the sea's surface near the shore 

 in a high wind, or in a gale ; and signifies the rapid succession of great waves that pass 

 to and strike the shore. 



