RAILWAY CONSTRUCTION. 229 



laying is carried on when the temperature is between 40 and 

 75. The results of very carefully conducted experiments show 

 that an increase of temperature of 1 F. will cause an iron or 

 steel bar, or rail, to expand or lengthen to the extent of seven 

 one-millionths of its length. Working this out for a range of 

 100 F. would give an increase in length of seven hundred one- 

 millionths, which would be equal to an extension of 2184 of 

 an inch in a 26-foot rail. For our home railways, therefore, a 

 space of -^ of an inch will be found amply sufficient to meet the 

 variations in length between the extremes of winter and summer, 

 for a rail from 26 feet to 30 feet in length. Too much allowance 

 for expansion is detrimental to the rails, because where the 

 spaces are excessively large the wheels drop into the hollow and 

 hammer or spread the ends of the rails. 



The fish-bolts should not be completely tightened up until 

 the permanent way is thoroughly set, and packed to its finished 

 line and level. 



On straight line the rail-joints should be laid square and 

 opposite to each other. Permanent-way laying with broken joints 

 is rarely adopted, except on curves or station-yards. 



On curves the joints of the inner rails gain on the joints of 

 the outer rails to the extent of 



radius + gauge 



T. ft X length of rail, 

 radius 



The amount of this gain, or lead, is adjusted by cutting off 

 a portion of the end of the inner rails at certain intervals. 



Assuming the fish-bolt holes to be spaced as shown on Fig. 

 342, then, when the inner rail is leading to the extent of 2 

 inches, a piece 4 inches long is cut off, as shown by dotted lines, 

 leaving the original second fish-bolt hole to serve as first or end 

 fish-bolt hole, and a new or second bolt-hole is drilled by hand 

 at A. This method sets back the joint 2 inches from the 

 square, and the lead is allowed to go on again until it becomes 

 necessary to cut off another piece of 4 inches. Another mode is 

 to have a proportion of the rails rolled 2 or 3 inches shorter for 

 use on the curves. 



On curves of a 1000 feet radius and upwards, the rails should 

 be laid to the normal gauge, but on curves of lesser radius the 

 gauge may be slightly increased, and as much as f of an inch 

 allowed on a curve of 500 feet radius. 



