If no compass is available, take two pieces of lead and place a few inches apart in a pot containing dilute sulphuric acid, scrape the lead clean, and join a piece of wire to each and connect to poles to be tested. After current has passed a short time one piece of lead will become brown, the other grey; trace the former to the dynamo cable, and this is the positive, and should be marked with a + or be painted red for future distinction. _Incandescent Lights or Glow Lights._ The number of lights required to illuminate any room would vary very much, according to the style of decoration and position of the lamps. As a rule, a similar number of glow lamps are required as there would be gas burners. The former give a much higher standard of illumination, which, curiously enough, is generally expected with electric lighting on account of the purity of the atmosphere when the full light is being used, which is not the case with gas. One 16 candle-power lamp will light an area of about 8 feet in diameter at 8 feet above ground; in ordinary situations allow, one lamp for 38 square feet. _Arc Lighting of Works._ External.—56. 2,000 CP. arc lights will illuminate 160,000 square yards, or one for each 2,800 square yards. Internal.—43. 2,000 CP. arc lights will illuminate 31,500 square yards, or one for each 730 square yards. _Approximate Cost of Electric Light, Museum._ Arc lighting, 2/5 gas; with interest, ⅔. Incandescent, ⅔ gas; with interest, 4/3. _Motive Power._ Compound engine 2 lbs. of coal per indicated horse-power per hour. Good single-acting engine 3 to 6 per indicated horse-power per hour. An indicated horse-power can be obtained in a compound engine from 20 lbs. of steam per hour. A good boiler evaporates 9 to 10 lbs. of water per lb. of coal. From 21 to 28 cubic feet of gas are required in a gas-engine per indicated horse-power per hour. _Incandescent Lamps._ In practice allow 9-60 watt 16 CP. lamps per indicated horse-power of engine. _Mem. for Wire Running._ Leads to the left or “low,” “_light coloured_.” Returns to the right or “raised,” “_red_.” _English and French Measures._ Millimetre = 0·039 inches 1 mill = ·0254 millimetres. Centimetre = 0·393 ” 1 inch = 2·5399 centimetres. Decimetre = 3·93 ” 1 foot = 3·3480 decimetres. Metre = 39·37 ” 1 yard = ·91439 metres. Cubic metre = 35·32 cubic feet or 1·31 cubic yards. ELECTRICAL MEASUREMENTS. The Paris Congress Units (1884) are now universally adopted and consist as follows: _Electro-motive Force, and Potential_ (E).—The Volt. _The legal volt is ·926 of the E. M. F. of a Daniell’s cell, which for rough purposes may be taken as a volt._ _Resistance_ (R).—The Ohm. The legal ohm is now represented by the resistance of a column of mercury of a square millimetre in section at the temperature of zero centigrade 1·062 metres long.