OF METEOEOLOGICAL INSTRUMENTS. 27 



79. Robinson's Anemometer, as No. 78, with extra dial extending the registration 

 to 5050 . . . . . . 550 



Robinson's Anemometer consists essentially of four hemispherical cups, having their 

 diametrical planes exposed to a passing current of air. They are carried by four horizontal 

 arms attached to a vertical shaft, which is caused to rotate by the velocity of the wind. Dr. 

 Eobinson found that the cups, and consequently the axis to which they are attached, revolve 

 with one-third of the wind's velocity, which is here measured by a simple arrangement of 

 two wheels, working in endless screws, and, by means of two indices, shows, on inspection 

 of the dial, the velocity of the wind. The outer, or front wheel, which revolves once for 

 every five miles, is furnished with two graduated circles, the interior circle being sub-divided 

 to miles and tenths of miles, whilst the outer circle is divided into 101 parts, each part being 

 equivalent to five miles, so that it measures 505 miles of wind. The stationary index at the 

 top of the dial marks on the inner circle the number of miles (UNDER FIVE) and tenths, that 

 the wind may have traversed, in addition to the miles shown by the traversing index, which 

 revolves with the dial and indicates on the outer circle the transit of every five miles. This 

 anemometer is rendered extremely portable by the arms which carry the cups being made 

 to take off. When in use it may be screwed on a shaft or ordinary piece of iron pipe 

 which accompanies it, and may be fixed in any desirable position, their construction being 

 such as to adapt them to withstand the most violent storms, and the simplicity of their 

 make enables the observer to clean and lubricate them at pleasure, twice a year being 

 sufficient. 



To Place the Anemometer, No. 78, and take the first reading- If after placing the 

 instrument the hands are at 0, the next observation will show the distance travelled by the 

 wind during the interval; but if the hands stand otherwise, then read them as they are, 

 by noting down the divisions and figures indicated by the traversing hand and stationary 

 index at the top. Thus, say that the former points to 125, and the latter to 2.6, making 

 together 127.6, this will now be the starting-point of the gauge. 



I. Example. Let the traversing hand point to any increased number on the outer 

 circle, say 375, and the stationary index to 3.6 ; these two numbers added together give the 

 true reading, i. e. (378.6) miles. From this reading 378.6 must now be subtracted the first 

 reading of the instrument, viz., 127.6, giving 251 miles as the distance traversed by the wind 

 during the interval. 



II. Example. Let the traversing hand now point to 425, and the stationary index to 

 4.7, adding these two together we have 429.7, from which take the last reading, viz., 378.6, 

 and the remainder, 51.1, will be the velocity of the wind for the interval between the two 



readings. 



III. Example. Lastly, let the traversing hand be at 175, and the stationary index at 

 2.8; here it is evident that the traversing hand, which at the last reading was at 429.7, must 

 have passed the highest number marked on the dial, viz., 505. Hence, to obtain the true 

 reading in this case, we must add together the three numbers, viz., 505, 175, and 2.8, together 

 making 682.8 ; from this, taking the last reading 429.7, we have 253.1 miles as the distance 

 travelled by the wind in the interval. 



To save time and dispense with figures, it is usual, when the traversing hand has passed 

 the 505, to place an asterisk at this point in the register, to denote that 505 must be added 

 to the next reading. 



To read the Anemometer, No. 79. The divisions and figures on the left-hand dial are 

 precisely the same as those in No. 78, and are read off in the same way. Each division on 

 the dial to the right represents 505 miles, the subdivision of which are shown on the left- 

 hand "dial. 



Example. Take the reading of both dials as they are found to be at the time, say the 

 left-hand dial showing 275.4 and the one to the right 505, and a little over a half; these 

 figures added give 780.4 as the true reading, from which must be deducted the former reading, 

 if any, to show the distance traversed by the wind in the interval. 



