PLEASURES OF SCIENCE. 25 



ing, have led, upon the pressure and motion of fluids, are of the 

 greatest importance, whether we regard their application to practical 

 purposes, or to the explanation of the appearances in nature, or their 

 singularity as the subjects of scientific contemplation. When it is 

 found that the pressure of water upon any surface that contains it, is 

 not in the least degree proportioned to its bulk, but only to the height 

 at which it stands, so that a long small pipe-full, containing a pound 

 or two of water, will give the pressure of twenty or thirty ton ; nay, 

 of twice or thrice as much, if its length be increased, and its bore 

 lessened, without the least regard to the quantity of the liquid : we are 

 not only astonished with so extraordinary and unexpected a property 

 of matter, but we at once perceive one of the great agents employed 

 in the vast operations of nature, in which the most trifling means are 

 used to work the mightiest effects. We likewise learn to guard against 

 many serious mischiefs in our own works, and to apply safely and 

 usefully a power calculated, according as it is directed, either to pro- 

 duce unbounded devastation, or to render the most beneficial service. 



Nor are the discoveries relating to the Air less interesting in them- 

 selves, and less applicable to important uses. It is an agent, though 

 invisible, as powerful as water, both in the operations of nature and of 

 art. Experiments of a simple and decisive nature show the amount 

 of its pressure to be between 14 and 15 pounds on every square inch ; 

 but, like all other fluids, it presses equally in every direction; so that, 

 though on our hand there is a pressure downwards of above 250 pounds, 

 yet this is exactly balanced by an equal pressure upwards, from the 

 air pressing round and getting below. If, however, the air be removed 

 below, the whole pressure from above acts unbalanced : hence the ascent 

 of water in pumps, which suck out the air from a barrel, and allow the 

 pressure upon the water to force it up 32 or 33 feet, that body of water 

 being equal to the weight of the atmosphere ; hence the ascent of the 

 mercury in the barometer but only 28 or 29 inches, mercury being be- 

 tween 13 and 14 times heavier than water. Hence, too, the motion 

 of the steam-engine ; the piston of which is pressed downwards by 

 the weight of the atmosphere from above, all air being removed below 

 it by first filling it with steam, and then suddenly cooling and convert- 

 ing that steam into water. Hence, too, the power which some animals 

 possess of walking along the perpendicular surfaces of walls, and even 

 the ceilings of rooms, by squeezing out the air between the inside of 

 their feet and the surface of the wall, and thus being supported by the 

 pressure of the air against the outside of their feet. 



The science of Optics, (from the Greek word for seeing,) which 

 teaches the nature of light, and of the sensation conveyed by it, pre- 

 sents, of itself, a field of unbounded extent and interest. To it the 

 arts, and the other sciences, owe those most useful instruments which 

 have enabled us at once to examine the minutest parts of the structure 

 of animal and vegetable bodies, and to calculate the size and the mo- 

 tions of the most remote of the heavenly bodies. But as an object of 

 learned curiosity, nothing can be more singular than the fundamental 

 truth discovered by the genius of Newton, that the light, which 

 we call white, is in fact composed of all the colours, blended in certain 

 proportions ; unless, perhaps, it be that astonishing conjecture of his 



