OF METEOBOLOGICAL INSTKUMENTS. 



25 



FIG. 75. 



ANEMOMETERS. 



The anemometers and air meters in the following list include only such as are 

 approved of by the highest authorities, and in constant use at the present time. The 

 table for converting velocity into force is introduced on account of the little confidence 

 usually felt in reports of the wind's force ; the authoritative course now followed being 

 almost exclusively confided to reports of velocity, from which the comparative force is 

 thus easily obtained. 



74. ANEMOMETEE (LIND'S), in which the pressure or force of the wind is shown by 



the depression and elevation of a column of water below and above the 

 zero of the scale (fig. 74) p. 31 . . . . 220 



75. Casella's Air Meter, for Mines, Hospitals, and other public buildings. The object 



of this little instrument is to give correct means of measuring the velocity 

 of currents of air passing through coal and other mines, and the ventilating 

 spaces of hospitals and other public buildings. It was first constructed for 

 Dr. Parkes, F.R.S., of the Eoyal Victoria Hospital, Netley, for measuring 

 the state of ventilation in that large military establishment, and declared 

 to bo the most perfect instrument of the kind in use. Since then it has 

 been adopted in our Houses of Parliament, the United States Senate, most 

 of our northern mines, and many of the leading prisons and hospitals 

 throughout the country. The graduations for each instrument are obtained 

 by actual experiment by means of machinery made for the purpose, so 

 that the indications of all are as comparable with each other as the weight 

 or measure of ordinary substances. The indications are shown by means 

 of the large dial and hand, and five smaller ones, as shown in the annexed 

 plate. The whole circumference of the large dial is divided into 100 parts, 

 and represents the number of feet up to 100 traversed by the current of 

 air. The five smaller dials are each divided into ten parts only, one 

 revolution of each being equal to ten of the preceding dial, and representing 

 1000, 10,000, 100,000, 1,000,000, and 10,000,000 respectively. By means 



