438 



METEOROLOGICAL INSTRUMENTS. 



velocity. A simple arrangement of wheels and screws is appended 

 to the instrument, which, by means of two indices, shows, on inspec- 

 tion, the space traversed by the wind. The outer or front wheel, one 

 revolution of which is equal to the transit of five miles of wind, is 

 furnished with two graduated circles, the interior being divided to the 

 eighth part of a mile, so that each division is equal to a furlong, 

 while the exterior is divided into one hundred parts, each being equal 

 to five miles. The stationary index at the top of the dial marks the 

 number of miles (under five) and furlongs that the wind may have 

 traversed, in addition to the miles shown by the traversing index, 

 which revolves with the dial and indicates the transit of every five 

 miles. The graduation is to five hundred. The traversing index is 

 furnished with a milled- headed screw at the back of the instrument, 

 which is employed for bringing its extremity to the zero point when the 

 instrument is set, which consists in merely turning it by means of the 

 milled-headed screw and bringing the end of the index to point to zero. 

 . By means of the folding arms which carry the cups this anemometer 

 is rendered portable. When in use it may be screwed on a shaft or 

 the ordinary piece of gas-pipe which accompanies it and elevated to 

 any desirable altitude. It is particularly adapted for occasional obser- 

 vations on shore, and is suitable for measuring the force of the wind 

 at sea. It may readily be set up on the highest part of a building or 

 elevated on board a vessel. When inspected it will show alike the 

 wind's present velocity as well as the rate at which it was passed 

 since it was set or last read. This instrument may also be used for 

 showing the ventilation of public buildings or dwellings, by an inspec* 

 tion of its dial in combination with a watch or clock, by which the 

 rate of the progress of ventilation may be seen. 



Fig~ 11. 



Fig. 1 1 represents a convenient form 



of Land's anemometer for showing the 



direction and force of the wind. This 



consists essentially of a glass tube of 



^k I i "/^Z ® half an inch bore, bent into the form 



of a U, as shown in the figure, the 

 lower half of which is filled with mer- 

 cury; the upper end of one of the legs 

 is bent horizontally, and when this is 

 directed toward the wind the mercury 

 is driven down by the pressure in one 

 leg and caused to rise in the other, 

 the difference in level gives the pres- 

 sure of the wind in inches of mer- 

 cury from which the velocity may be 

 calculated. For observing very high 

 winds the straight leg may be closed 

 at the top, in which case the pressure 

 on the open end will be indicated by 

 the condensation of the air in the 

 other leg, combined with the difference 

 of level of the mercury in the two 

 leffs. 



