1843.J 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



273 



a = o; d = 26057. 

 A = 1 ; D = 26056. 

 That is, the pressure at the surface will be equal to a column of air 

 of uniform densrty 26057 feet high ; and consequently one foot above 

 the surface = 26056 feet high, or a foot less. Since the densities 

 are as the pressures, we have 



26057 

 A - a = 1= - rl °E- 26056 ; 



making 26057 = /(, we have 1 = x log -— — 



but log, 



«— 1 \« 



+ 



l 



+ 



+ 



i 



fee. 



J 



W ' in 



And when, as in the present case, n is a large number, all the terms 



but the first may be neglected as unimportant; also, since M = 



•43429448, the modulus of decimal or common logarithms, 



. , -43429448 



•• 1 = - rX -2605T 



26057 

 whence x = „ 42q .» = 60000, very nearly, or x may be readily 



found from the expression x — 



The above formula is reduced to 



A = 60000 log. -g feet ; 



or putting m and M the heights of the murcury at the earth's surface 

 and at the altitude A, then the fraction 



d _ m 



T> ~ M 

 Also since 6 feet are equal to one fathom, the simple multiplier 60000 

 for feet becomes 10000 for fathoms, which is more convenient. Hence 

 instead of 



A = 60000 log. — feet 



We have A = 10000 log. ^ fathoms, 



which is the formula formerly used in measuring altitudes by the ba- 

 rometer. 



With respect to the height taken for the homogeneous columns of 

 air different writers vary, but this difference does not affect the ulti- 

 mate result. It is well established, that the height of an homogeneous 

 atmosphere, whose density would be equal to that of the air at the 

 earth's surface, and weight the same as that of the real atmosphere 

 when compared with a column of mercury or other fluid of the same 

 weight, the heights will be reciprocally as the specific gravities of 

 the air and mercury or other fluid. So that if we take the specific 

 gravity of the air at the earth's surface at lj, when compared with 

 distilled water at 1000, and that of mercury 14000, also the column 

 of mercury in the barometer = 294 inches, we have 1^ : 14000 '. ' 

 29& ; 344166 inches = 28680-5 feet = 5-43 miles. But the specific 

 gravity of fluids varies as their temperatures vary. It has been found 

 by various experiments, that when the mercury in the barometer 

 stands at 30 inches, and the thermometer at 55°, the specific gravity 

 of air, water and mercury are nearly as 1^, 1000 and 13600. Hence 

 H : 13600 : : 30 : 340000 inches = 28333i feet = 5-366 miles, 

 the height of a homogeneous atmosphere. Again, taking the specific 

 gravity of the air at the earth's surface at f, which some affirm, and 

 the barometer at 294 inches, it will be If : 13600 ; ; 294 : 328255 

 inches = 27318 feet = 5-1814 miles. Hence, generally, we may 

 assume if the air was of the same density at all altitudes as at the 

 earth's surface, its height would be between five and six miles. But 

 it matters not what degree of temperature we assume, for we can 

 always accommodate the result to any other temperature, as before 

 observed, by augmenting or diminishing the result by the ^tb part 

 for every degree above or below 31°. 



It may further be observed, that the common barometer, with some 

 trifling alterations, of which we shall speak hereafter, is the best and 

 most to be depended on ; for many which are said to be improved 

 have only the recommendation of deviating from it in simplicity. 

 It appears from accurate observations, that mercury stands higher in 

 tubes of larger than in those of narrower bore ; and therefore, when 

 observations are made with different barometers, attention should be 

 paid to the difference of their diameters. In order to prevent the 

 effects of the attraction of cohesion, the bore of the tube should not 

 be less than one fourth of an inch ; but one third of an inch would be 

 better. 



Note — Further particulars relative to this important subject is 

 given in a work about to be published, entitled " A New Theory of 

 the Heavens and Earth." 



OBSERVATIONS ON THE CONSTRUCTION OF ROADS 

 THROUGH BOG. 



The bogs in Ireland form a vast extent of surface, which for the 

 most part, is profitless, save for fuel, but a large portion of which is 

 capable of being converted at a small expense into good arable land, 

 which would yield an average crop ; but the absence of roads in these 

 extensive wastes, has prevented the capital of individuals from being 

 profitably invested, in the reclamation of those barren plains, now 

 tenanted by the plover and the snipe. 



It seems to have been the system with engineers of former days, to 

 avoid by every means the construction of a road across bog, and accord- 

 ingly they have not scrupled, by a very circuitous route, greally to 

 extend the distance between the termini in preference to encountering 

 the unstable foundation of a bog, which the science of road-making 

 has, in latter years, rendered a comparatively easy and safe under- 

 taking. Still, even in modern works, it is to be regretted that so fre- 

 quently is to be seen a road, otherwise well selected and judiciously 

 laid out, materially injured by deviating from its proper course, in 

 order to avoid an intervening bog ; and as the engineer, in tracing a 

 line of public road, should ever bear in mind that the chief advan- 

 tages of communication is to open the country, and to afford every 

 facility for improving and drawing out the resources of those lands, 

 whose capabilities have been suffered to lie dormant for want of this 

 advantage, the intersection of bogs by roads should therefore not be 

 avoided. 



The most favourable months in the year for commencing operations 

 in the construction of bog roads, are June, July, August and Sep- 

 tember (if dry), since at those periods the surface is more free from 

 water, and consequently the bog more firm than in the rainy months ; 

 and it is of the greatest importance to have fine weather for these 

 works. 



When a line of road passes over a shallow bog of from three to 

 four feet in depth, with a firm and compact substratum, and when 

 soling material could not be procured without the expense of land 

 damage, and the cost of carriage should be great, or when the soling 

 should be of an inferior or unsuitable quality, it will be most advan- 

 tageous (provided the gradients will admit) to remove the bog mould 

 altogether, to form the road upon the hard bottom, and to sink the 

 grips sufficiently deep to prevent the moisture of the adjacent bog 

 producing injurious consequences. 



In the construction of a road over the surface of a bog, the iirst 

 point to be attended to, and which is of the chief importance, is the 

 thorough drainage of that portion of the bog upon which it is deter- 

 mined to place the road materials. When the line is lockspitted, a 

 grip is to be cut on both sides of the road of sufficient capacity to 

 command the complete drainage of the surface ; and in soft bog, a 

 second grip should be cut on both sides, at about 10 feet distance from 

 the first ; and it will be particularly necessary that great attention 

 should be observed in keeping them free from impediments, and that 

 there should be no obstruction to the free passage of the water. The 

 second grip, in addition to the greater facility of drainage, will he 



37* 



