24 EFFECTS OF WINDS AND OF 



Assumption No. 3 is merely an approximation adopted to simplify the 

 computation. The rate at which any barometric gradient changes varies 

 continuously in general. The errors in the final computed results introduced 

 by assumption No. 3 are believed to be small. Such errors are discussed 

 later in an appropriate context. 



The preceding statement in regard to the derivation of the form of the 

 observation equations for the least-square solutions which determine the 

 barometric constants is written primarily with reference to Lake Erie. The 

 only modification necessary to make the statement appropriate for Lake 

 Michigan-Huron is that indicated on pages 15-16, where it is shown that for 

 Lake Michigan-Huron the barometric gradients are to be taken from ex- 

 pressions (14) and (15) rather than from expressions (12) and (13), which 

 were used on Lake Erie. The corresponding changes must be made in 

 equations (16) and (17). For the locations of the points 3, 4, 5, and 6 used 

 in expressions (14) and (15), see page 15 and plate 2. 



EXAMPLE OF OBSERVATION EQUATIONS FOR BAROMETRIC 



EFFECT. 



The form of the observation equations for a least-square solution to deter- 

 mine barometric effects is that shown in equation (30), page 20, which is 

 here repeated for convenience as equation (40) : 



b^ w +b m B m +b^ m +b m B m +b m B m +b m B ni -\-b nt B nt +b n B nt +I=V (40) 



The meanings of the separate terms in the equation are also repeated here 

 for convenience. 



The current day is defined as the day to which the equation is assigned in 

 listing the equations. 



For Lake Erie : 



5 w =(6-8) at 8 p.m. on the day before the day preceding the current 



day minus (6-8) at 8 a.m. on that preceding day. 

 b m = (6-8) at 8 a.m. of the preceding day minus (6-8) at 8 p.m. of that 



day. 

 b^ (6-8) at 8 p.m. of the preceding day minus (6-8) at 8 a.m. of the 



current day. 

 ^=(6-8) at 8 a.m. of the current day minus (6-8) at 8 p.m. of the 



current day. 



The quantity (6-8) is the barometric pressure at the point marked 6 on 

 plate 2 minus the barometric pressure at the point marked 8 on that plate. 

 The barometric pressure is expressed in inches of mercury. 



For Lake Michigan-Huron, use (4-5) instead of (6-8) ; see plate 2. 



The appropriate modification for any other lake is evident. A point to 

 the westward of the lake is to be used in the place of the point 6, and one to 

 the eastward and in the same parallel is to be used in the place of point 8. 



