326 
Tangent to two Circles . 
[Oct, 
e greatest strain. It may be said that this beam is supported further by the 
earns, shewn in the sections through C and D, and so it is ; but the strain on 
bear the 
cross beams, 
both is in a transverse direction, and they also, for the reason above stated, would 
be more effective in the bottom of the boat. 
In the old system of roofing, I believe one of the greatest faults was a want 
of harmony (if I may be allowed the expression) , in its several parts, for while 
some were so placed as to allow of that “settling down,” as workmen term it, which 
takes place more or less in all roofs, others resisted it, and thereby deranged the whole. 
Now I may be mistaken, but something of this sort appears to me conspicuous in 
Captain Forbes’ plan ; for allowing the centre of gravity of the engines and 
boiler to be at the section C, any “ sagging” down of the bottom of the boat 
of that point, would pull upon the bolt which passes through C, and cause a de- 
flection on the truss beam B D, in which state it would act as a double lever 
upon the points B and D ; e being one fulcrum and f the other : that this would 
take place I am not prepared to say, but it seems clear that there is nothing but 
the tranverse resistance of the timber to prevent it. 
Should you consider these remarks of sufficient interest to obtain a place in your 
work, they may serve to correct an erroneous opinion, which has gone abroad, that 
the boat which has been built is after my plan ; and this opinion would seem to rest 
upon no better grounds than simply because the Government at one time so ordere 
it. Should you be desirous of any further information on this subject, I shall wi 
ingly furnish you with a copy of the several opinions of the members of the Com- 
mittee who sat in judgment on the two plans. 
Diamond Harbour, Sept.lGlh, 1831- _ * . _woT 
Note by the Editor.— The remark alluded to by Captain Cowles, did not i J 
refer to the plan and elevation of his model of a steam tug, but to the raioin 
Captain Forbes’ tug, which appeared to us on reading the work, to ditter io ^ 
specification of the same boat in App. A 3, in having only one longi u 1 
beam instead of a pair of parallel beams made on the same principle, w 
pleasure however in giving insertion to the exposition of our correspon 
of construction, which pleased us much in the model, and presented no 
ciple, except in the slight unavoidable curvature inwards of the tl USS f 
bow and stern, and in the transverse timbers which, if we mistake not, torn P 
quadrilateral figures. 
VIII. To draw a Tangent to two circles . Plate XIX. fig- 
A good deal having appeared in the public prints lately, respecting the Hi ^ 
College, it may not be uninteresting to publish the accompanying so u ^ 
Geometrical Problem by one of the pupils there, Radha Nath Sikdar. . 
is altogether his own discovery, and I have not altered a word in his con^> 
Let DMG and HNC be two circles, it is required to draw a tangent th ^ uiD 
Let A be the centre of the circle GMD, and B of CNH : join A . 1V 
the point L in such away that AL may be to LB as AM to NB. gC 
From L draw LD, touching GMD in the point D ; join AD ; throug i > ^ 
parallel to AD, and let it meet DL produced in C. Then DC is the 
Because the triangles ALD and BLC are similar, AL . LB . • 
and also by construction AL : LB : : AM : 
Whence BC=nd C is in the circumference of the circle NCH. 
Because CB is parallel to AD, the angle BCL is a right angle. 
Hence DC is the tangent to both the circles. 
