182 FERGUSON'S LECTURES. 



LECT. compared with water, it is mixed with some lighter 

 ^?^s metal, such as tin. 



By this method, the specific gravities of all bodies 

 that will sink in water, may be found. But as to those 

 which are lighter than water, as most sorts of wood are, 

 the following method may be taken, to shew how 

 much lighter they are than their respective bulks of 

 water. 



Let an upright stud be fixed into a thick flat piece of 

 brass, and in this stud let a small lever, whose arms are 

 equally long, turn upon a fine pin as an axis. Let the 

 thread which hangs from the scale of the balance be tied 

 to one end of the lever, and a thread from the body to 

 be weighed, tied to the other end. This done, put the 

 brass and lever into a vessel ; then pour water into the 

 vessel, and the body will rise and float upon it, and 

 draw down the end of the balance from which it hangs : 

 then, put as much weight in the opposite scale as will 

 raise that end of the balance, so as to pull the body 

 down into the water by means of the lever ; and this 

 weight in the scale will shew how much the body is 

 lighter than its bulk of water. 



There are some things which cannot be weighed in 

 this manner, such as quicksilver, fragments of diamonds, 

 &c. because they cannot be suspended in threads ; and 

 must therefore be put into a glass bucket, hanging by a 

 thread from the hook of one scale, and counterpoised 

 by weights put into the opposite scale. Thus, suppose 

 you want to know the specific gravity of quicksilver, 

 with respect to that of water ; let the empty bucket be 

 first counterpoised in air, and then the quicksilver put 

 into it and weighed. Write down the weight of the 

 bucket, and also of the quicksilver ; which done, empty 

 the bucket, and let it be immersed in water as it hangs 

 by the thread, and counterpoised therein by weights in 

 the opposite scale : then, pour the quicksilver into the 



