256 



REPORT — 1844. 



N.B. The mean of S.S.E. is the mean difference of S.S.E. and N.N.W. 

 These values being laid off geometrically as a system of mechanical forces, 

 and as shown in figs. 13 and H, Plate XXXVIII., we arrive at the following 

 deductions, showing the general results of Osier's anemometer at Devonport. 



The direction of the current, as thus obtained, is, it must be admitted, 

 rather different from that given by the observations with Professor Whewell's 

 instrument, yet the mean velocity is not far different. This disagreement in 

 respect of direction, although to a certain extent unsatisfactory, is perhaps 

 not more than we might expect would happen in the present imperfect state 

 of such instruments. It must, I think, be admitted that the direction given 

 by Whewell's anemometer is too far north ; and this error would probabl}' 

 arise from the circumstance that the machine does not do really what it pro- 

 fesses to do, viz. trace a line always proportional to the space passed over 

 by a particle of air in a giver» time. The revolutions of the fly, in fact, are 

 not always proportional to the velocity of the wind, except in very strong 

 breezes, as we have already shown in Table XL; hence the weak westerly 

 and northerly winds, so very prevalent in this place, and of which the vane 

 of Osier's instrument has left traces, have not produced their full effect on 

 Mr. Whewell's instrument, especially as compared with the generally strong 

 southerly and easterly vvinds, which keep the Ry in rapid motion ; hence an 

 undue prevalence in the records of these winds. On the other hand, the 

 mean force of the north and west currents, so frequent in the records of 

 Osier's anemometer, has probably been taken too high, it having been deter- 

 mined without reference to the frequency of the light winds which prevail 

 from these quarters; and this, together with the failure of the instruments in 

 I'egistering all the strong southerly gales, has thrown the direction too far 

 south. The mean direction of the wind at Devonport, as determined by the 

 spaces which a particle of air would have passed over at the end of one year 

 in each of tlie given directions, will probably be found eventually from south- 

 west to north-east ; at all events the limits of their direction is between north 

 and east. 



The Astronomer Royal, with his usual kind consideration for those en- 

 gaged in scientific pursuits, has been so good as to place in my hands the 

 very interesting volumes of the Greenwich Meteorological Observations for 

 the years 1841 and 1812, by which I have been enabled to institute a com- 

 parison of the Plymouth with the Greenwich observations. 



By the results given in pages 47 and 53 of the vol. for 1841, and in pages 

 78 and 85 for 18*2, we are enabled to treat the results of Osier's instrument 

 at Greenwich in a way similar to that just shown for Plymouth. Deducing 

 the mean forces for the recorded observations, and the total hours of each 

 wind from the two-hourly observations, we have similar elements to those 



