424 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



Dkc 



EMBEK, 



perpendicular, iiuich less resist the slightest force of wind ; and I need 

 scarcely add how little adapted it must be for the accuracv which the 

 iiitlior appears to have sought in its construction. 



Again: I l.elieve it is generally admitted that the chief deside- 

 ratum in a levelling instrument is a steady and secure basis, indepen- 

 dent as far as possible of the wind, or any accidental movement. 

 Now the ball-and-socket joint introduced by Mr. S, on which the 

 whole weight of the level must rest cannot be considered so tirni and 

 secure as the common table-plate usually employed, and I have invself 

 had more than one instrument of the usual construction pass through 

 my hands, which, from not being originally well balanced and centred, 

 has been considerably atl'ectcd in its adjustment by a simple revolution 

 on its axis — the additional weight of metal given to one side probably 

 acting unequally on the bearing surface at ditlerent points of the cir- 

 cuit. How greatly nuist the liability of such derangement be in- 

 creased when the support is thrown on the rounded surface of a ball, 

 instead of on a strong horizontal plate. 



Tlie independence of the nature of the ground and the actual po- 

 sition of the legs, in setting up the stand is, however, a decided im- 

 provement in ilsci/, in the plan of Mr. Stevenson, both as regards the 

 economy of time and labour, ami the usual wear and tear, and conse- 

 quent irregular action of the levelling screws under tlie con)inou form. 

 In a level, which was submitted to the notice of the Institution about 

 a month before that of Mr. 8., I had endeavoured to obtain the same 

 object by placing a small circular spirit bubble in the head of the 

 stand itself, by which this may be brought to an approximate level, 

 previously to the instrument being placed upon it, and this may be 

 done upon any ground where there is standing room to use it. But 

 at the same time a wide and steady base was deemed of so much im- 

 portance, that I more than doubled the usual levelling surface of the 

 plate, by adopting a modification of the tripod-stand for the G inch 

 theodolites used by Col. Everest in the Indian survey. 



Having had some experience in the use of engineering and astro- 

 nomical instruments, I have ventured to trouble you with these ob- 

 servations, not for the sake of depreciating the ingenuity of Mr. Ste- 

 venson's improvements, but because I deem it of some importance to 

 the profession that we should not be led to mistrust the accuracy of 

 the instruments in common use, and thus to refine on points which in 

 practice become non-essential, whilst we overlook what is of far more 

 importance for correct results — the handling of the tools on which all 

 our operations depend ; 



And I am. Sir, 



Your's, &c. &c., 



Yarmouth, Nor. 13. George Tow.nsend. 



ON MEASURING DISTANCES BY THE TELESCOPE. 



By Edmund Bowman. 



[The following very interesting paper was read at the last meeting 

 of the British Association. We consider it well deserving the con- 

 sideration of the Profession, as such we have given the paper without 

 abridgment. — Editor.] 



Many years ago having had the charge of a level belonging to a 

 celebrated engineer of the present day, while assisting him in taking 

 levels, &c. for a bridge over a river in the north of England, my 

 curiosity was excited (being quite a boy at the time) to know the 

 utility of a narrow transparent scale in the field of view of the telescope, 

 and which I afterwards conjectured was for the purpose of measuring 

 distances. Having a few years afterwards procured a telescope level, 

 and having made a reading staff, at that time quite a novelty, I tried 

 a few experiments to find out what proportion, if any, existedbetwecn 

 the distance of the object and the diameter of the field of view, when 

 1 found that, after the first 100 feet, the distances were nearly in pro- 

 portion to the diameter of the field of view, read oft' from the reading 

 staft'held at these distances from the level — this might be in the year 

 1830; it occurred to me that the reading stall; when properly divided 

 into feet, tentlis, and hundredths, furnishes, by its image in the focus 

 of the object-glass of the telescope, a much more correct micrometer 

 scale, than any screws or a slip of any transparent substance can fur- 

 nish. For instance, the diaphragm or focal aperture of a 20 inch 

 telescope is -39 inches, and at the distance of 10 chains it covers 13S0 

 parts or hundredths of a foot, or 13 and S-lOths feet; now, each of 

 these parts is distinctly perceptible, or at least appreciable, but to 

 divide i-.'Sths of an inch into 13^0 parts, so as to be equally accurate 

 and visible, and the figures likewise equally distinct, would, 1 think, 

 be no common task, be the artist who he might, and the substance 

 divided, what it would; but could even such a thing be done, the 



reading-stall' would still be preferable (and in any case it would be ne- 

 cessary to have a stalFj as will appear on the perusal of this paper. 



In the year IS 10, about Midsummer, having procured one of Trough- 

 ton's best levels, with a 20 inch telescope, I made several experiments 

 with it with the intention of finding out, by means of careful observa- 

 tions, the exact relation which the diameter of the field of view bore 

 to the distance measured, taking care, while making these observations, 

 to keep the eye-piece to one mark for viewing the image of the staff 

 and cross-lines stretched over the aperture of the diaphragm, and to 

 make the adjustment for focal distance of the object-glass^ as correct 

 as the eve could appreciate ; and having carefully observed the results, 

 and with these results, and interpolating between them, having made 

 a table of distances with their corresponding diameters or readings, I 

 then, with the level and staff alone, took observations, connected for 

 many miles, and after reducing the same by the above tables into dis- 

 tances, and summing up the whole, measured over the same ground 

 with the chain, when the agreement between the chain measure and 

 the telescopic measure was found very near, the difference not exceed- 

 ing t'le Trnrt'i P^'"' "^ the whole distance; and as part of this might 

 possibly arise from the rating of the telescopic measure, and the other 

 part from the inequalities of tlie ground, tlie truth might lie between 

 these two methods. 



I have also tried this method with correct surveys done many years 

 ago, and have found it correct; field for field, fence for fence, all in 

 their correct places, and over many miles in direct distiince. 



Now, since it would be very convenient, if the tables could be dis- 

 pensed with, and if each reading of the staft' either gave the actual 

 distance or a proportion that could be determined — with this view I 

 afterwards took a series of observations, with measured distances as 

 before, but with a smaller instrument, beginning with the nearest limit 

 of distinct vision, and proceeding by degrees to a distance of Jths of a 

 mile; then compiled a table as before, taking care to place a mark to 

 all those readings and distances which were actually observed and 

 measured ; upon comparing and examining these results, and those of 

 the former table, I obtained the idea that the readings can all be made 

 to bear a certain proportion to the distance by adding to each, be the 

 same great or small, a certain fixed quantity or constant, peculiar to 

 each instrument. The superior simplicity of this operation renders 

 the tables now no longer of any use. 



The cause of the inequality of the readings is the aberration of focus 

 arising from the object or radiant point being at different distances, 

 and the object-glass itself not being at the centre of the station. At 

 first it occurred to me that it might be possible to enlarge and contract 

 the diaphragm containing the image by some contrivance, with the 

 view of keeping the image and the diameter of the diaphragm always iu 

 proportion to the inverse ratio of the distance, or what may perhaps be 

 generallv better understood, that the angular amount of the field of 

 view might remain the same for all distances: but this has the objec- 

 tion, that the screws or levers by which this contracting and expand- 

 ing would be cff'ected, would be liable to get out of order, from wear 

 and other causes, and no longer perform accurately, and even if they 

 did so, the readings would only be in proportion to the distance from 

 the object-glass, and not from the centre of the station or instrument. 



By fixing the diaphragm and the object-glass at one invarialile dis- 

 tance, and producing distinct vision for the various distances by having 

 a lens or part of the object-glass williin the telescope, moveable by 

 screw adjustment two or three inches to or from the object-glass, all 

 objects beyond a distance of 10 or 20 feet might have their images 

 produced at the same distance from the object-glass ; this method also 

 would require reduction to the centre of the station as in the previous 

 case, and moveover the achromatism of the object-glass would only be 

 good at one point of its motion, and in no case be so good as with an 

 united or cemented object-glass. 



But these two methods and others have been rejected for the more 

 simple and convenient one mentioned above, and which the following 

 paper proposes (o explain mure amply, both as regards its theory and 

 practice, and which method is simply to add a fixed qu intity to each 

 reading of the staff to make the said readings proporlional to their 

 respective distances, a small reading having thus an equal increase 

 with a large one; the objection that might be raised to this method is 

 this, that the accuracy of it evidently depends upon the distance be- 

 tween the object-glass and the focus or image of the object at the 

 diaphragm being jnecise, and as the telesco|)e, without alteration, is 

 used as it is, this point might admit of reasonable doubt, :is to extreme 

 accuiacy ; but these doubts have been entirely removed by the method 

 of determining the exact focal point for any distance, thus setting the 

 matter at rest with respect to accuracy, and leaving nothing more to 

 be desired. 



For this method of determining distances, the telescopes upon the 

 levels, as at present constructed, are quite sufficient ; but if this meets 



