NOTES AND QUERIES.

Otago Witness, Issue 2727, 20 June 1906, Page 47

NOTES AND QUERIES.

Questions for reply in coming ittut to &• received not UUr than MONDAY night.

A. M., Maori Point. — Mr H. M. Davey, cousulting engineer, 91a Princes street, replies- — I must apologise to you for not answering your question with the extreme exactitude that I now learn from your second letter, you require. My -error came from thinking that you were a practical man needing a practical answer, but this now appears not to be the case. If you remember, I said your txmnel would be half full when the wate rrose to 2ft Sin full, and I have yet to learn tha-t that answer was wTong, considering the wording of your first letter. Now, however, I have submitted the whole for another way of working oat, to JCx Ivor Da^fy. Ist sec , 8.A., of Wellington, who kindly replies by post, as follows: — Yonr figure is taken as a trapezium and lettered »t the angles B C A D, E F being the surface line to be found, A being the height of this line, or from the bottom to the surface of the •stater, and i being the line itself, or the surface of the water, the height of which is required. Now the area or the lower half or A D F E + th« upper half E F C B = B C A D, and tbat is 45 square feet in area. Al«o area of A D F X = area of X F C.B: i c..

suppose you merely want the n.b_p. to settle some argument, for otherwise it is not of much use, and fortunately is becoming less so. Taking up two works on the quest-ion you send, the only information they give is just as follpws: — " N.h.p. is a term deservedly falling into disuse. It was at one time employed by makers to denote only the size of the engine, without reference to the power it might develop." Another way to put it would probably be that when low pressures and moderate speeds were nearly universal the size of the cylinder did really give some rough idea of the power, and so it was said that a single lOin cylinder gave about 10 horse nominal power, an Bin being less than 8 h.p., and say a 12in being more than 12 h.p. Now what with the great increase in both pressures and speeds, all rules for finding the n.h.p. get further and further from the truth, till, as Molesworth observes, the actual power in some engines is over six times the nominal power. The power is now best found either by the indicated horse-power or the brake horsepoweT. Both these have their advantages. The indicated, when properly taken, shows if anything is wrong, and, if so, what, to a great extent ; while the brake c»n be made to give not only the power of the engine, but of each or all of the machines it is driving as well ; and you can get the net power that the engine and shafting combined has after the friction in driving

and either out of 3am c roughly form a bearing with a bolted cap capable of being tightened on said shaft, or else secure a suitable bearing capable of being so tightened, to said lever. I might mention that Where the power is considerable a drum or pulley could advantageously often be substituted for the shaft, as it does not matter how or where you place the drag as long as you can begin to reduce the fuil speed slightly. The lever, as above, with a piece of old belting which would resemble the letter P, makes often a good brake. When the brake is made, and the lever is balanced, which can be done by, say, a ba" of sand, etc., a spring balance or scale and weights, though the former is handiest, is secured at or near the end of the lever at a convenient distance from the centre of the shaft, and when provision in the shape of buffets, etc., so that the lever shall not cause mischief, is placed all is ready. Now turn on the motor with the brake on the slisift or dram n*odera.tely loose, and when the usual full speed is arrived at tighten the bearing or brake till the motion is slightly slowed, and take the number of revolutions per minute and the number of pounds on the balance. The amount of slowing does not matter, as Jf you slow more you most have more power on the balance, and the reverse. You will have measured the distance from the centre of the shaft to the centre line of the pull, or where the spring balance is. tied, either