50 LECTURES TO SCIENCE TEACHERS. 



tific weighing, in which the greatest accuracy is required, that 

 allowance must be made not only for the different force of 

 gravitation at different latitudes, "but also at places differing 

 more or less in height from the sea level. That the force of 

 gravitation at different points of the earth's surface is also 

 affected by the density of the earth underneath such points 

 is beautifully exemplified by Dr. Siemens' instrument, the 

 bathometer, as will be explained to you by himself. 



5. With these preliminary observations upon the subject 

 of weight, I will now proceed to the subject of balances, the 

 more immediate object of this lecture. 



The balance in its simplest form is a beam made to vibrate 

 upon a centre of motion, with pans, or other contrivances for 

 supporting bodies weighed, hanging from the extremities of 

 the two arms of the balance. From the depression of either 

 pan the excess of weight of the body placed in it is deter- 

 mined. Balances may be classed under two heads : (1) Ordi- 

 nary balances, or scale-beams with equal arms, having the 

 beam, suspended at its middle. If an equal-armed balance 

 is properly adjusted so that the beam is exactly horizontal 

 when the pans are empty, the balance will be in equilibrium ; 

 and the balance will also be in equilibrium, that is to say, 

 the beam will rest in a horizontal position, after equal weights 

 have been placed in the pans. (2) Balances with unequal 

 arms, in which the beam vibrates upon the centre of motion 

 placed more or less near one of the extremities. These two 

 classes comprehend all balances of precision, or scientific 

 balances. Spring balances are also used, where the body 

 weighed pulls down a spring to which a pointer is attached, 

 moving on a graduated scale, and thus indicating the weight. 

 But these balances are not balances of precision. 



6. You may here see a good specimen of an equal-armed 

 balance, made by Mr. Oertling, who has constructed most of 

 the best balances of the Standards Department. It is made 

 to hold a kilogram, or a 2 Ibs. weight, in each pan. You may 

 see that when the balance is set in motion, the pans being 

 empty, the beam is exactly horizontal. In order that it 

 should be so, it is necessary that the frame of the balance 

 should be accurately levelled ; and you may see that is done 

 by two spirit levels placed upon it. The horizontal position 

 of the beam is shown by the pointer at one of its extremities 

 being in the middle of the graduated index. 



