860 REPORT—1899, 
and similar segments on the spindles of these gear into racks on the three telescopic 
legs, thus compelling them to radiate outwards or inwards simultaneously, 
Secondly, the centre pin is arranged to receive a small drawing board to attach 
a sheet of paper to. 
Thirdly, this centre pin also forms a pivot on which to mount a telescopic 
pencil arm, having a roller at its outer end, the pencil being near its inner end. 
This arm being moved round on the pivot, the roller will move inwards or outwards 
if the furnace is deformed, and the pencil, following the movements of the roller, 
and describing a small circle, will also move inwards or outwards, thus delineating 
on the paper the deformities full size, but, as these are shown on the small circle, 
they will apparently be greatly magnified, and so may easily be read by the eye, 
thus enabling the commencement of deformities to be easily detected long before 
it would be otherwise observed; for if the roundness of a furnace is once 
destroyed, the defect accentuates itself under ordinary working conditions. The 
possibility of the gradual, but at last probably excessive, if not also dangerous, 
distortion of furnaces might be guarded against, and the cause removed, if only 
the first sign of the circularity of the furnace being defective were discovered. 
The diagram will completely delineate the actual shape of the furnace around 
the whole of its circumference, and so enable the boilermaker, when setting a 
furnace into truth, to know exactly where to deal with these defects. 
These diagrams should be taken when furnaces are new, before and after the 
hydraulic test is put on, and retained for future reference when furnaces are from 
time to time examined with this gauge. 
The instrument itself was shown. 
4. Experiments on the Thrust and Power of Air-Propellers, 
By Witut1am Greorce Watker, JIE, A.M. 
The first set of experiments were made with air-propellers of 2, 3, and 4 feet 
in diameter. The following laws were proved for top speeds up to 15,000 feet 
per minute. 
The thrust varies as the square of the number of revolutions, also as the area. 
The horse-power varies as the cube of the revolutions, and for small angles as the 
square of the angle of pitch. In the case of the four-foot diameter air-propeller, 
a thrust of 15 lbs. per horse-power, at a top speed of 15,000 feet per minute, was 
obtained. 
Experiments were then carried out on air-propellers of 30 feet in diameter. 
They were made as light as possible, and weighed about. 150 Ibs. each, designed 
for giving a thrust of 1,000 lbs. The area of the four blades was 360 square feet. 
At forty revolutions per minute, a thrust of 120 lbs. was obtained with about four 
horse-power. A Mangin type of propeller was employed, the blades being fixed 
one behind the other, and connected together by diagonal struts and ties; the 
object in placing a blade immediately behind avother one is to increase the 
strength and stiffness of the blades. The blades are made of solid drawn steel 
tubes, and of diameters varying from inch to 1 inch. 
The 30-foot air-propeller was tried at progressive revolutions, varying from 
10 to 100 revolutions; also at different angles, varying from 4° to 20°, 
