889.—Anemometers are used by mining engineers for testing the ventilation of mines. The original and best known form is that of Biram, Fig. 431. This instrument is held in any current of air, and the velocity of the current is registered by the motion of oblique fans, by means of ordinary decimal gearwork on five dials giving feet and multiples by 10. Lowne's anemometer, with the author's improvements, Fig. 432, is of similar principles of construction, but it is arranged in portable form to go in a pocket case. Another well-known form of anemometer is built upon the same principle, but of cubical form. It is customary to take the velocity of the current for one minute by a watch, there being a detent provided in most instruments to start and stop the motion of the hands upon the dials.
890.—Books of Tables and Formulæ.—Few British Surveyors are without Molesworth's pocket-book. This contains all the useful tables and notes of reference valuable to the civil engineer in his ordinary work—weight, 5 ozs. Many pocket-books have been written on the same plan. Hurst's pocket-book contains all matters of reference for the town surveyor among buildings. Trautwine's Civil Engineer's Pocket-book (American) is the most complete, but it is of double the weight of the Molesworth. Spon's Engineers' Tables for the Waistcoat Pocket—weight, little over ½ oz.—is a very useful little book. Of Traverse Tables both Gurden's and Boileau's are comprehensive and reliable. There are several pocket-books of Curve Tables, those of Cutler & Edge, Beazeley, and Kennedy & Hackwood being perhaps in the most general use.
891.—Technical Books—Ordnance Maps are published on special districts and subjects which are often relative to the country or the special conditions of work abroad and at home for minerals, etc. It is very useful to possess such of these as may be required, and the note is only made here as a reminder.
892.—Sling Case for Drawings.—The most convenient method of carrying maps or drawings for public works in execution is to have a solid leather case similar to a telescope case. This is best if made with the cap or lid of the same length as the body: it can then be drawn out any distance according to the length of the rolled drawing. If thought more convenient, and the map or drawings are heavy, a strap may be added to pass over the shoulder, Fig. 433.
893.—Chronometer.—This may be any form of watch with compensated escapement. At present the prices run high for this class of work; but from the simplicity and moderate certainty of compensation it does not appear that this should be necessary for the production of a fair working instrument useful for the surveyor in new countries to check his longitude. Where a good chronometer is used it is better to keep it to Greenwich time without alteration. If there is a gaining or losing rate this will most probably remain constant in equal times, so that corrections may be made pro ratâ for all observations until a check can be taken with certainty when arriving at a town which possesses an observatory. The quality of a chronometer is fully ascertained by having a certificate from one of our observatories, that of Kew being the most popular.
Fig. 433.—Sling case for drawings.
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894.—Chronograph.—For the observation of stars in transit for the purpose of taking longitude, a dead-stop watch or chronograph is most useful. This can now be had in combination with an otherwise fair going watch at a very moderate price.