TiiK srxniAr. axp 'nn% clock. 107 



Ami we must add the i.-([iKili()ii with the minus si^ii to dial 

 time it" the cU)ck be slow. 



To those who do not happen to be aware of it, — to add 

 with the minus si^ii is to subtract. 



We have now reached the proper place for a remark. 



The Medo-American who compares watch with dial will 

 be as much disappointed as the Parisian of a few moments 

 ago unless he remember to take account not only of time 

 Equation l)ut of a fatal 92 seconds which separate our local 

 mean solar time from the mean solar time of 75° west of 

 Greenwich. 



In order to a due understanding of why this is so a word 

 or two al)out Standard Time will be in order. Elementary 

 enough to some, but not to all. 



If correctly kept, mean solar time varies with every 

 meridian at the rate of one hour of time to every fifteen 

 degrees of arc ; four minutes of time to every degree of arc, 

 and four seconds of time to every minute of arc. For exam- 

 ple, in our latitude, say 40° north, let it be mean solar noon 

 at any place, — here where we are if you choose. It will then 

 be four seconds after noon at a place 1552 yards along the 

 parallel to the eastward ; or one second after noon at 388 

 yards to the eastward : and equally of course one second before 

 noon by the watch at 38S yards to the westward along the 

 parallel. The general fact is the same in any latitude, but the 

 number of yards would be different, for reasons easily found 

 in the proper place. 



The two extremities of the United States, Quoddy Head 

 in Maine and Attn Island in Alaska are separated by 121° 8' 

 of longitude ; being equivalent to 8h 4m 32sec of time. 



The instant of absolute time, the same for the whole uni- 

 veise, is recorded as one thing in one place and another thing 

 in another. 



To lessen the inconvenience arising from these necessarily 

 differing records the mean solar time of some suitable meri- 

 dian is used over a wide tract of countrv. In England the 



