42 POPULAR LECTURES AND ADDRESSES. 



temperature at which it is to be used ; but the 

 main quality required for good work is constancy 

 in temperature of maximum rate, and in co- 

 efficient for calculating rates at other temperatures. 



To facilitate the application of Hartnup's 

 method at sea, a small thermometer, to be placed 

 in the chronometer case, with a scale graduated 

 not to degrees but to squares of numbers of 

 degrees of difference from the temperature of 

 maximum rate, would be a valuable adjunct to be 

 supplied to every chronometer. The navigator in 

 winding his chronometer daily, would look at this 

 thermometer, and enter two or three figures in a 

 properly prepared chronometer rate-and-reckoning- 

 book. All that he would have to do, thus, to take 

 full advantage of Hartnup's method, need not 

 occupy more time than about as much as it takes 

 him to wind his chronometer. 



30. INSTRUMENTS FOR MEASURING SPEED 

 AND DISTANCE RUN. The name log was origin- 

 ally applied to a floating piece of wood, by the 

 aid of which the speed of a ship through the 

 water was determined. What is commonly called 



