1818.] Col. Beaufoy on the Spiral Oar. 249 



chord of this circle. To calculate the radius sufficiently accurate 

 for practical purposes, add to three times the length of the arc 

 the diagonal of the cylinder ; from -i- of the sum add and subtract 

 half the diagonal of the cylinder, then multiply the sum and 

 difference of these numbers together, and the square root of the 

 product is the versed, sine of the arc. Next divide the square of 

 half the chord by the versed sine, to the quotient add the versed 

 sine, and half the sum is the radius of that part of the metallic 

 curve nixt the cylinder. Besides giving the inner and outer 

 edge in' the metal a circular form, a second operation becomes 

 necessary, which is hammering the plate to elongate the outer 

 edge, beginning at the part next the cylinder, and gradually 

 proceeding to the outer edge. Without this operation the me- 

 tallic leaf which forms the spiral would not, when fixed, be 

 perpendicular to the cylinder. This part of the work being so 

 difficult that few workmen can execute it well, it depends on 

 science to invent a machine, perhaps by passing the metallic 

 leaf between conical rollers, which, by pressure, might supersede 

 the necessity of hammering, and would render this instrument 

 of more practical utility. In conclusion it must be remarked, 

 that it was originally intended that the axis of the wind spiral 

 should have measured 18 inches, but the difference, half an inch, 

 was designed to allow for the friction of the machine and the 

 resistance of the air to the radii. For the purpose of determin- 

 ing if half an inch was a sufficient allowance for the impediments 

 the spiral met with when Uuning,.an upright and revolving shaft 

 was erected, from which projected an horizontal arm, to the 

 extremity of which was fixed the frame of the wind spiral, the 

 upright shaft being turned a given number of revolutions : the 

 space the extremity of the arm moved, calculated and compared 

 with the distance shown by the spiral, the difference was as 

 1000 to 779, that is, the spiral in moving 10U0 yards gave the 

 distance too small by 221 yards : a table being constructed, the 

 true velocity may be had by inspection. 



Trusting some of your readers will not deem the subject 

 unworthy their consideration, I remain, Gentlemen, 

 Your obedient humble servant, 



Mark Beaufoy. 



Article IV. 



Memoir relative to the Annular Eclipse of the Sun, tohich will 

 happen on September 7, 1820. By liancis Baily, Esq. 



{Concluded from p. 186 ) 



Having thus given a general outline of this eclipse, I shall 

 proceed to state the principal phenomena which have been 



