666 THE POPULAR SCIENCE MONTHLY. 



haps, better than many regulators do ; but, while it does not make an 

 average variation of a second a day, it is far enough from making an 

 actual variation of less than a second a day. Something better is ob- 

 tainable from the very best astronomical clocks, which, indeed, are 

 found to keep a uniform rate, from which they will vary only three 

 or four hundredths of a second daily. But astronomical clocks, as is 

 well known, are not required to indicate the exact time, mean or side- 

 real, but only to go at a uniform rate, which, if it be found to be 

 practically invariable, is corrected at the time of the observation in 

 which it is employed. The most accurately running large clock in 

 the world, which has been regulated to keep diurnal time, is the 

 "Westminster clock in England. When the contract for the build- 

 ing of that clock was given out, it was stipulated that it must come 

 within a second a day. It, in fact, does much better than this, for it 

 is found that its variation is usually less than a second a week. It 

 telegraphs its time daily and automatically to Greenwich, and the 

 astronomer royal has said of it, " The rate of the clock is certain to 

 much less than a second a week." 



The other practical method of regulating the escape of a time- 

 piece is through the balance-wheel, which, of course, must be resorted 

 to in case of watches, ship-chronometers, and all clocks which are to 

 be moved or carried about. 



The method of regulation now in use in the watch is the result 

 of long study of ingenious men. Starting with the discovery of 

 the balance-wheel, carried back and forth with a diminishing oscil- 

 lation by means of the hair-spring, we have before us the question 

 of how to unlock the scape-wheel with each swing of the balance. 

 In hunting through all classes of watches which find their way into 

 the jeweler's refuse-box, after having served out their period of 

 usefulness, we shall not be likely to come upon more than three 

 (and certainly not more than four) styles of escapement. The old- 

 est watches in the box will have what is termed the horizontal or 

 barrel escapement. This is the escapement of the so-called " bull's- 

 eye " watch of our grandfathers. These watches always had a great 

 reputation as time-keepers, yet I presume it is safe to say that there 

 never was one in existence which could be relied upon to keep within 

 a minute a day in the pocket, and most of them needed a much larger 

 allowance. This escapement had a small wheel, with its axle parallel 

 with the plates of the movement, and its teeth acted upon the pallets, 

 which were little, flag-like appendages to the staff of the balance, and 

 set at right angles to each other. The chief defect of it was, that a 

 slight variation in the power, acting directly as it did upon the bal- 

 ance, affected very materially the rate of the watch. So that, while 

 our grandfather's clock was even a better time-keeper than clocks of 

 the same grade manufactured to-day, our grandfather's watch is not 

 to be named in comparison with the cheapest modern watches. 



