68 



ON THE GENERAL DIFFUSION OF KNOWLEDGE. 



rammed down, or by cutting a trench EF, about 

 a foot and a-half wide, along the bank of the river 

 or canal, and a little deeper than the bottom of 

 the canal; which being filled up with earth or clay 



well moistened with water, forms, when dry, a 

 kind of wall through which the water cannot pe 

 netrate. By inattention to such circumstances 

 many disasters have happened, and much ex 

 pense needlessly incurred; and, therefore, the 

 scientific principles to which I have now adverted 

 ought to be known, even by labourers of the 

 lowest rank employed in operations carried on 

 for the improvement of the country. 



To the want of a recognition of these prin 

 ciples may be attributed (he failure of the follow 

 ing scheme, and the disaster with which it was 

 attended : After the diiing-bell was invented, 

 it was considered desirable to devise some means 

 of remaining for any length of time under wa 

 ter, and rising at, pleasure without assistance. 

 &quot; Some years ago, an ingenious individual pro 

 posed a project, by which this end was to be ac 

 complished. It consisted in sinking the hull of a 

 ship made quite water-tight, with the decks and 

 sides strongly supported by shores, and the only 

 entry secured by a stout trap-door, in such a 

 manner, that, by disengaging from within the 

 weights employed to sink it. it might rise of itself 

 to the surface. To render the trial more satis 

 factory and the result more striking, the projec 

 tor himself made the first essay. It was agreed 

 that he should sink in twenty fathoms water, and 

 rise again without assistance at the expiration of 

 24 hours. Accordingly, making all secure, 

 fastening down his trap-door, and provided with 

 all necessaries, as well as with the means of mak 

 ing signals to indicate his situation, this unhappy 

 victim of his own ingenuity entered, and was 

 sunk. No signal was made, and the time ap 

 pointed elapsed. An immense concourse of 

 poople had assembled to witness his rising, but 

 in vain ; for the vessal was never seen more. The 

 pressure of the water at so great a depth had, no 

 doubt, been completely under-estimated, and the 

 sides of tUt* vessel being at once crushed in, the 

 unfortunate projector perished before he could 



even make the signal concerted to indicate his 

 distress.&quot;* 



Many other applications of the principles of 

 hydrostatics might have been mentioned, but 

 what has been now stated may serve to exemplify 

 the practical utility of an acquaintance with such 

 principles, not only to engineers and superinten- 

 dants of public works, but tj mechanics and arti 

 ficers of every descriplion. 



The science of Pneumatics, which treats of 

 the mechanical properties of the atmosphere, 

 will likewise be found useful to mechanics and 

 artists of various descriptions, to whom it is, in 

 many cases, of importance to know something 

 of the effects of the resistance, the pressure, and 

 the elasticity of air. The construction of baro 

 meters, syphons, syringes, and air-pumps, de 

 pends upon the pressure of the atmosphere, and 

 likewise water-pumps, fire-engines, and many 

 other hydraulic machines ; and, consequently, 

 the constructors of such instruments and en 

 gines must frequently act at random, if they are 

 unacquainted with the nature and properties of 

 the atmosphere, and the agency it exerts in such 

 mechanical contrivances. f Even the carpenter 

 and the mason may be directed, in some of their 

 operations, by an acquaintance with the doc 

 trines of pneumatics. When two pieces of 

 wood are to be glued together, they are first 

 made as even and smooth as possible ; the glue 

 is then applied to one or both of the surfaces ; 

 they are then pressed together till the glue has 

 become thoroughly dry. &quot; The use of the glue is 

 to fill up every crevice in the pores of the wood, 

 so as to prevent the admission of any portion of 

 air between the pieces ; and then the atmos- 



* Herschel s &quot; Discourse on the Study of Nal. 

 Philosophy.&quot; 



t As an illustration of the importance of being 

 acquainted with the atmospheric pressure, the fol 

 lowing anecdote may be here inserted : A respecta 

 ble gentleman, of landed property, in one of the 

 middle counties of Scotland, applied to a friend of 

 mine, a Lecturer on Chymistry arid Natural Philo 

 sophy, in order to obtain his advice respecting a 

 pump-well which he had lately constructed at con 

 siderable expense. He told him, that, notwithstand 

 ing every exertion, he could not obtain a drop of 

 water from the spout, although he was quite sure 

 there was plenty of water in the well, and atih&amp;lt;,ugK 

 he had plastered it all armtnd, and Hocked v.p tvtry 

 crevice. &quot;When my friend inspected the pump, he 

 suspected that the upper part of the well was air 

 tight, and, consequently, that the atmospheric pres 

 sure could not act on the surface of the water in the 

 well. He immediately ordered a hole to lie bored 

 adjacent to the pump, when the air rushed in with 

 considerable force ; and, on pumping, the water flow 

 ed copiously from the spout. The gentleman was 

 both overjoyed and astonished ; but It is somewhat 

 astonishing, that neither he, nor his neighbours, nor 

 any of the workmen who had been employed in its 

 construction, should have been able to point out the 

 cause of the defect ; but, on the other hand, should 

 have taken the very opposite means for remedying 

 it, namely, by plastering up every crevice, so as to 

 produce a kind of vacuum within the well. This 

 and similar facts show how little progress scientific 

 knowledge has yet made, even among the middle 

 classes of the community 



