\ 



2^ . J^otes on the Design of Telescope Tubes. 



being that, while the bridge girder is required to resist forces 

 in one plane only, the telescope tube is, by condition 5, 

 required to resist forces in various planes. 



The effect of the force of gravity upon each particle of 

 material in the telescope may be resolved into two portions 

 —one along the length of the tube, the other at right-angles 

 to its length. The first of these will attain its maximunii 

 value when the tube is vertical, and will vanish when it 

 becomes horizontal ; the second will attain its maximum when 

 the tube is horizontal, and will vanish when it is vertical. 

 The effect of the first set of forces will be to shorten or com- 

 press the tube longitudinally, thus bringing the specula 

 nearer together. But this result is not a practical evil ; for 

 it is, in the first place, excessively minute, and, further, is 

 completely neutralised by the action of focussing the instru- 

 ment. The second set of resolved parts — those at right- 

 angles to the length of the tube — tend to bend the tube, 

 and thus throw the specula out of their proper relative posi- 

 tions opposite each other; this is a more serious evil, as it at 

 once impairs the action of the optical part of the instrument. 

 In designing our tubes, we need therefore have regard 

 only to forces at right-angles to its length. 



A properly- designed framed girder for a bridge will be 

 found almost invariably to consist of two massive parallel 

 straight members or booms, connected together by a system 

 of more slender straight bars, forming with the parallel booms 

 a system of triangles. The essential conditions of strength 

 and stiffness are in this case — 1st, that the structure should 

 consist of an assemblage of triangles ; the triangle being the 

 only polygon the form of which is absolutely fixed when th^ 

 length of its sides is known, and therefore the only figure 

 which will maintain its shape in spite of external forces 

 without requiring its various parts to endure a cross-bending 

 action ; • and 2nd^ that all the sides of the triangles should be 

 straight, for seeing that they are called upon to endure 

 longitudinal compressions and tensions alone, a crooked or 

 curved form is plainly inadmissible. No one would think 

 of making a pillar intended to carry a heavy load^ or a ti^^ 

 rod to endure a heavy tension, other than straight. 



Now, our framed telescope tube, like the bridge girder, 

 must consist of a series of rectilinear triangles^, and it must 

 also have its massive longitudinal booms. Two booms will 

 not now, however, suffice, for no longer are the forces we have 



