■348 Description of a new Anemometer. 



from it is by far tlie most violent, but comiiinnly of a 

 ijiuch shorter duration and extent. An accuraie measure 

 ot the velocity of wind has lono; been sought by meteoro- 

 logists : several have been devised on the Contfineiit, but 

 only two, that I know of, in England. That whicii I 

 now lay before the Acaclemy seems to me to be the sim- 

 plest and best adapted to the purpose. 



1 . The force of wind, lo winch the degrees of its velocity 

 are proportional, is measured by that of gravity indicated in 

 pounds and parts of a pound avoirdupois ; the calculation is 

 grounded on the observations of Mr. Smeaton, m the 

 Philosophical Transactions, vol. li. p. Ifio,. 



2. Mr. Smeaton indeed observes, that the evidence of the 

 velocity is not so great where this exceeds 50 nnles, a.-! when 

 50 or under; yet, from its agreement with other observa- 

 tions, Tam inclined to think it fully sufficient. 



3. A velocity of 123 feet per second was observed at 

 Petersburgh, an. 1741, 3 Much. 468, that is, at the rate of 

 83'S miles per hour. 



4. According to Lalandc, in his Treatise on Navigation, 

 42 Roz. Jour. 221, the course of the trade winds is be- 

 tween 6 and 7 miles an hour. 



5. Mr. Brice, Philosophical Transactions, 1 756, p. 226, 

 observed a storm whose velocity was 63 miles per hour. 



6. A fair wind at sea, is that whose velocity amounts to 

 20 feet per second, or 13*63 miles per hour. Ibid. 



7. Bonguer found the velocity of winter storms to be 

 about 34 miles prr hour, and in sum.mer nearly 43. Ibid. 



The distance from Holyhead to the Pigeon- house is 70 

 miles; tlien supposing the wind to be duect, and its velo- 

 city 30 miles per hour, and if we suppose the packet-boat 

 to assume 0*4 of the velocity of the wind, it will arrive a^ 

 the Pigeon-house in 5-3 hours. 



Let W denote the velocity of wind in the open air, or 

 meeting no opposition ; 

 D = the distance of the place towards which the 



wind tends ; 

 N the number of hours it requires to traverse that 

 distance ; 

 Then any two of these being known, the other may be 

 found by the followino formulas. 



Thus if W=30, D = 70 

 then N = ^-^- = 2-33 



D = 30X2-33 = 70 



\V = J.}^ = 30. 



A well- 



