430 



Procecdinga of the Royal Irish Academy. 



pied "svith eleterminirLg equations for ^"oltman's Fly, and Wilde's Wind 

 Tablet. Too niucli praise cannot be given to the ability an.d care with 

 wMch. tbese experiments were carried out, and it is not from any 

 failure in these respects that they caniLot be depended on to give reli- 

 able values of Anemometer constants, or correct the theoiy of these 

 instruments ; but they are all liable to several causes of uncertainty. 

 Even had the apparatus been established in fi-ee air, it is probable 

 that the Anemometer's indications "would be different from those given 

 by the same instrument if stationary, and acted on by a cuiTent of 

 wiad. Experiments shoTv that a plane surface moved thi'ough water 

 in a direction perpendicular to itself is less pressed than when a 

 stream of water impinges on it with the same velocity. I know Colonel 

 Duchemin's researches only by M. Dohrandt's reference to them, 

 and his results seem excessive; but De Buat's expeiiments give 

 that the ratio is as 1'186 : 1 ; Yince mates it 1"2 : 1. The pressure is 

 also differently distributed over the surface in the two cases. It is 

 possible that if the plane were moved down the stream with a less 

 velocity than it has, the difference might be still greater. I am not 

 acquainted with any experiments of this kind made on curved surfaces. 

 Similar differences may be expected to exist between the motion of the 

 body and that of the fluid in elastic media, though we cannot say 

 what would be theii' amount, supposing them to exist, though probably 



Stow's No. 0, ?- = 9, d = i: inelies, with. Xowiioff for V nearly equal, and taking 

 V = \ oi Ms number, as aU tlie instruments register Zv, I find — 



Casella. 



Stow. 



Casella. 



Dohr. 



No. 5. 



Stow. 



Nowikoff. 



Casella. 



Robinson. 



V 



F' 



i ^' 



V 



V 



F' 



F' 



V 



V 



F' 



71 



F' 



F' 



34-8 



3-936 



34-19 



3-163 



36-0 



3-696 



33-31 



3-225 



21-96 



3-263 



26-2 



3-85o 



25-94 



3-275 



i 24-7 



3-681 



27-29 



3-347 



16-00 



3-564 



227 



3-895 



21-16 



3-380 



i 21-8 



3-644 



22-37 



3-361 



11-40 



3-654 



20-2 



3-816 



20-27 



3-573 



16-3 



3-645 



15-98 



3-463 



10-58 



3-861 



16-2 



3-754 



15-31 



3-599 



i 7-9 



3-619 



7-74 



3-957 



9-79 



4-140 



11-0 



3-765 



11-12 



3-635 











7-10 



4-170 



7-4 



3-768 



















They are also inconsistent with my own observations, made many years ago, to 

 compare a Casella with my own Anemometer ; the results of which I give. I re- 

 gret that I was not acquainted with lli'. Stow's experiments during their progress, 

 as I would have requested him to measm-e for each of his instniments my con- 

 tants 3° ' -f b, and /. A knowledge of these might have modified his numbers 

 considerably. He seems to have not duly appreciated the extent to which very 

 slight modifications of the figure of the ground will influence the velocity of the 

 wind, and the considerable variations of it which occur even at small lateral dis- 

 tances. From my own observations of these facts I cannot place any reliance on 

 this mode of determiniug Anemometer constants. 



