62 



must preferably proceed. If the density be equal on all 

 sides, as in some confined tracts, a hurricane happens : hence 

 the advantage of ascertaining and comparing the degrees of 

 its velocity; for those being known, its cause and degrees of 

 rarefaction may with great probability be inferred. Two 

 causes of rarefaction are already known, solar heat, and some 

 internal chemical action, by which a quantity of air is con- 

 verted into water, and sometimes even into a stony sub- 

 stance ; this last being the most sudden and complete, the 

 rarefaction of the neighbouring air arising from it, is by far 

 the most violent, but commonly of a much shorter duration 

 and extent. An accurate measure of the velocity of wind 

 has long been sought by meteorologists : several have been 

 devised on the Continent, but only two, that I know of, in 

 England. That, which I now lay before the Academy, seems 

 to me to be the simplest and best adapted to the purpose. 

 The force of wind, to which the degrees of its velocit}' are 

 proportional, is measured by that of gravity indicated in 

 pounds and parts of a pound averdupois; the calculation is 

 grounded on the observations of Mr. Smeaton, in the Philo- 

 sophical Transactions, Vol. LI. p. 165. 



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

 velocity is not so great where this exceeds 50 miles, as when 

 50 or under ; yet, from its agreement with other observations, 

 1 am 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 

 83,8 miles per hour. 



4. According 



