THE CHRISTIAN PHILOSOPHER. 



Hydraulics, which has sometimes been treated 

 as a distinct department of mechanical philoso 

 phy, may be considered as a branch of hydrosta 

 tics. It teaches us what relates to the motion of 

 fluids, and how to estimate their velocity and 

 force. On the principles of this science, all ma 

 chines worked by water are constructed as 

 steam-engines, water-mills, common and for 

 cing pumps, syuhons, fountains, and fire-en 

 gines. 



III. Pneumatics. This branch of philosophy 

 treats of the nature and properties of the atmos 

 phere, and of their effects on solid and fluid bo 

 dies. Frori this science we learn, that air has 

 weight, and presses on all sides, like other fluids ; 

 that the pressure of the atmosphere upon the top 

 of a mountain is less than on a plain beneath ; 

 that it presses upon our bodies with a weight of 

 several thousand pounds more at one time than 

 at another ; that air can be compressed into forty 

 thousand times less space than it naturally occu 

 pies ; that it is of an elastic or expansive nature, 

 and that the force of its spring is equal to its 

 weight ; that its elasticity is increased by heat ; 

 that it is necessary lo the production of sound, 

 the support of flame and animal life, and the ger 

 mination and growth of all kinds of vegetables. 



These positions are proved and illustrated by 

 such experiments as the following : The gene 

 ral pressure of the atmosphere is proved by such 

 experiments as those detailed in No. II. of the 

 Appendix. The following experiment proves 

 that air is compressible. If a glass tube, open 

 at one end, and closed at the other, be plunged, 

 with the open end downwards, into a tumbler of 

 water, the water will rise a little way in the tube ; 

 which shows, that the air which filled the tube 

 is compressed by the water into a smaller space. 

 The elasticity of air is proved by tying up a 

 bladder, with a very small quantity of air within 

 it, and putting it under the receiver of an air- 

 pump, when it will be seen gradually to inflate, 

 till it becomes of its full size. A similar effect 

 would take place, by carrying the bladder to the 

 higher regions of the atmosphere. On the com 

 pression and elasticity of the air, depends the 

 construction of that dangerous and destructive in 

 strument, the air-gun. 



That it is capable of being rarified by heat, is 

 proved by holding to the fire a half-blown bladder, 

 tightly tied at the neck, when it will dilate to 

 nearly its full size ; and if either a full-blown 

 bladder, or a thin glass bubble filled with air is 

 held to a strong fire, it will burst. The elasti 

 city of the air is such, that Mr. Boyle, by means 

 of an air-pump, caused it to dilate till it occupied 

 fourteen thousand times the space that it usually 

 does. That air is necessary to sound, flame, 

 animal and vegetable life, is proved by the fol 

 lowing experiments : When the receiver of an 

 air-pump is exhausted of its air, a cat, a mouse, 

 or a bird, placed in it, expires in a few moments, 



in the greatest agonies. A bell rung in the same 

 situation produces no sound ; and a lighted cundl* 

 is instantly extinguished. Similar experiments 

 prove that air is necessary for the flight of birds 

 the ascent of smoke and vapours, the explosion 

 of gunpowder, and the growth of plants ; am 1 

 that all bodies descend equally swift in a place 

 void of air; a guinea and a feather being found 

 to fall to the bottom of an exhausted receiver al 

 the same instant. 



On the principles which this science has es-^ 

 tablished, have been constructed the air-pump, 

 the barometer, the thermometer, the diving-bell, 

 the hygrometer, the condenser, and various other 

 instruments, which have contributed to the com 

 fort of human life, and to the enlargement of our 

 knowledge of the constitution of nature. 



IV. Acoustics. This science treats of the 

 nature, the phenomena, and the laws of souna, 

 and the theory of musical concord and harmony. 

 From the experiments which have been made on 

 this subject, we learn, that air is essential to the 

 production of sound ; that it arises from vibrations 

 in the air, communicated to it by vibrations of 

 the sounding body ; that these vibrations, or 

 aerial pulses, are propagated all around in a 

 spherical undulatory manner ; that their density 

 decreases, as the squares of the distances from 

 the sounding body increase; that thev are pro 

 pagated together in great numbers from different 

 bodies, without disturbance or confusion, as is 

 evident from concerts of musical instruments ; 

 that water, timber, and flannel, are also good 

 conductors of sound ; that sound travels at the 

 rate of 1142 feet in a second, or about thirteen 

 miles in a minute ; that the softest whisper flies 

 as fast as the loudest thunder; and that the ut 

 most limits, within which the loudest sounds 

 produced by artificial means can be heard, is 

 180 or 200 miles;* that sound striking against 

 an obstacle, as the wall of a house, may, like 

 light, be reflected, and produce another sound^ 

 which is called an echo; and that, after it has 

 been reflected from several places, it may be 

 collected into one point or focus, where it will 

 be more audible than in any other place. On 

 these principles, whispering galleries, speaking 

 trumpets, and other acoustic instruments, are 

 constructed. 



V. Optics. This branch of philosophy treats 

 of vision, light, and colours, and of the various 

 phenomena of visible objects produced by the 

 rays of light, reflected from mirrors, or trans 

 mitted through lenses. From this science we 



* In the war between England and Holland, In 

 1672, the noise of the guns was heanl in those parts 

 of &quot;Wales which were estimated to be two hundred 

 miles distant from the scene of action. But the 

 sounds produced by volcanoes have been heard at a 

 much greater distance ; some instances of which 

 are stated in Chap. IV. Sect. 2. Several otfier facts 

 in relation to sound are detailed in Chap. III. Art 

 Acoustic Tunnels. 



