“For example, if the first dial points to 7, or between 7 and 8; while the second dial indicates 4; and the third, 5; the entry to be made is 745 (indicative of 745 thousand revolutions).
“Every time the index of the first dial is found to have passed zero (0), a cross or star is to be prefixed to the next (a lower) reading.
“To ascertain how many thousands of revolutions have been made during the month, it will simply be necessary to subtract the first reading from the last, and prefix to the three figures thus obtained a figure corresponding to the number of stars in the column. For every thousand revolutions there are two miles of wind: we have therefore only to multiply by 2 to find how many miles of wind have passed during the month.
“Two entries must be made for the last day of each month (the one being written under the other), so as to bring the readings down to 9 A.M. on the 1st of the following month. The same entry which ends one month, will therefore begin the next. This repetition of one entry is necessary, in order to prevent losing a day’s wind.
| “The accompanying example of the | 687 | ||
| readings of an Anemometer for 13 days | 773 | ||
| will illustrate the method of making | 822 | ||
| the entries, &c. | 855 | ||
| “In this instance, the first read- | 900 | ||
| ing (687) is less than the last (793). | 953 | ||
| When the first reading is greater than | 990 | ||
| the last, it will be necessary to borrow | *066 | ||
| 1,000 in making the subtractions, | 197 | ||
| and then deduct one from the number | 323 | ||
| of stars. Thus, if the first reading | 414 | ||
| of the series on the margin had | 597 | ||
| been 887, the result would have been | 712 | ||
| 906 instead of 1106. | 793 | ||
| 1106 | thousands of revolutions. | ||
| 2 | |||
| 13 | 2212 | miles of wind in period. | |
| 170 | miles of wind per day, on an average. | ||
“The foregoing directions are all which require to be regularly attended to. But it may be interesting at times to find the velocity of the wind during a period of a few minutes. This may be ascertained by observing the difference of two readings of all the dials, with an interval of some minutes between them, when a very brief calculation will suffice; but perhaps the simplest method is the following:—
Fig. 88.
“Take two readings, with an interval of 12 minutes between them. The difference of these readings, divided by 10, is the velocity of the wind in miles per hour. Thus—if the reading of the five dials (from left to right) at noon is 15206, and at 12 minutes past 12 is 15348, the velocity of the wind is 14·2 miles per hour.”—Admiral FitzRoy, F.R.S.
A lever and clutch are sometimes fitted to this anemometer, as in fig. 88, for throwing the train out of gear when not required to register. It may also be connected with clock-work so as to be self-recording, by causing the mechanism to impress a mark upon prepared paper moved by the apparatus, at certain intervals of time.