ON HTDROSTATIC INSTRUMENTS, AND HYDRAULIC ARCHITECTURE. 309 



usual, and on the other a loop of fine wire, or of horse hair, for holding the 

 solid to be weighed, which may be changed occasionally for a ball of glass, 

 suspended in a similar manner: sometimes also a dish is added for holding any 

 loose substances which will sink in water, proper counterpoises being used as 

 equivalents for the weight of the dish either in air or in water; and when a 

 body lighter than water is examined, a weight of known magnitude and 

 density is employed for sinking it. (Plate XXI. Fig. 277.) 



The specific gravities of any substances, and in particular of such as are 

 lighter than water, may also be very conveniently determined by means of a 

 common balance, employing a phial with a conical ground stopple, filling it 

 first with water, and then either with a given fluid, or with a portion of the 

 solid of which the weight has been ascertained, together with as much water 

 as is sufficient to exclude all the air. 



For the speedy examination of a variety of fluids, differing but little in 

 specific gravity from some known standard, a hydrometer may be very con- 

 veniently employed. This instrument is said to have been invented by Ar- 

 chimedes: it consists of a hollow ball, with a weight below it, and a slender 

 stem above, so graduated as to express the specific gravity of the fluid by the 

 decree to which it sinks. Sometimes the instrument is sunk to a certain 

 mark, by means of weights placed in a dish at the end of the stem; or different 

 weights are fixed to it below, while the graduations of the scale are still ob- 

 served ; and it may even be applied to finding the specific gravities of solids, 

 the solid being first placed in the dish at the end of the stem, and then in a 

 second dish which is suspended from the bulb below the water. (Plate XXJ, 

 Fig. 278.) 



Another mode of ascertaining the specific gravities of fluids differing bu<. 

 little from each other in density, is to have a series of globules of glass, so 

 loaded as to correspond to the specific gravities indicated by as many numbers, 

 which are marked on them ; and, throwing several of them together into the 

 fluid, to observe which of them remains nearly stationary, without either 

 vising to the surface or sinking. This method, though not expeditious, ap- 

 pears to be very secure from error: the globules are sold by patent, adapted 

 for the measurement of the strength of spirituous liquors. 



kV 



