The Rev. T. R. Robinson's Description of an improved Anemoimter. 157 



which, I may add, seems to follow a different law of velocity from the plus one. 

 In LiND a similar uncertainty is produced by the negative action of the wind 

 on the remote aperture of the gauge. 



3. It has been well observed by Forbes, in his Second Report of the British 

 Association on Meteorology, that little progress can be made in anemometry, 

 except by the employment of self-registering instruments. If these record pres- 

 sure, we cannot thence readily deduce the mean velocity, even admitting the 

 law F- = mP. Let V and E' be their mean values ; V + c, P'+ ir, any others ; 

 n their number, then 



nV- + 2F' X S{v) ■vS{if) = nmF + mS{-n), 



or as S(v) and SM = 0, o/ 2^ 



n 



in which the last term is often of very great magnitude ; or if we take 



T^' + ,. = //« X VP, 

 we have 



V = \/m X : 



71 



but I have found the trouble of computing the sums of the square roots, even 

 for a few minutes, an insuperable objection. 



These seem to me sufficient reasons for absolutely rejecting the pressure- 

 gauge, and adopting instead of it one which gives directly the velocity, or 

 rather its equivalent, the space traversed in a given time. Instruments fulfilling 

 this object are by no means of recent date. One was described in 1749 by the 

 Russian Lomonosoff ; it consisted of a vertical wheel with float-boards hke an 

 undershot, half of which was screened, and which was kept in the plane of the 

 wind's direction by a vane. This, by a train of wheel- work, indicated on a dial 

 the revolutions of the wheel ; there was no provision for recording these in 

 connexion with time, but a very ingenious one for noting the quantities of 

 wind which blow from each point of the compass. A much neater one was 

 constructed in 178.3, by the late Mr. Edgeworth, and used by him to measure 

 the velocity of air ciurents, though designed for a different purpose.* It con- 



* For measuring the ascent of a baDoon ; two years later it was used by our countryman 

 Ceosbie in his perilous ascent, and was preserved by him when the rest of his apparatus was lost 



t2 



