302 Mr WHEWELL'S RESULTS OF OBSERVATIONS 



or eighteen inches high. By this means the surface of the cylinder 

 would contain the trace of the wind of one or two days when there 

 was much wind, and of several days when the winds were lighter. 



These instruments were made at first by Mr Newman of Regent 

 Street, and since, by Mr Simms of Fleet Street. They have been 

 erected and observed by Professor Forbes and Mr Ranken at Edin- 

 burgh, by Mr Southwood at Plymouth, and also at other places; but 

 the observations of whicli I am able to give the most complete account 

 are those made under the direction of Professor Challis at the Cambrido-e 

 Observatory, and under my own direction at the house of this Society. 

 The Anemometer at the Observatory was placed over the portico, in 

 which situation it was free on the other sides, but considerably sheltered 

 by the dome of the equatoreal, on the north side. The Anemometer 

 placed on the top of the Society's house is favourably circumstanced, 

 being higher than any neighbouring building which is near enough to 

 intercept the wind. The observations were made with care and regu- 

 larity by Mr Crouch the Society's housekeeper. 



Various improvements in the instrument were suggested by using 

 it ; and as it had not been foreseen what strength of workmanship would 

 be requisite to resist the weather, all the instruments were, at one time 

 or other, disabled, so as to interrupt the observations. 



One of the difficulties which most interfered with the precision of 

 the observations, was that which arose from the icaveting of the wind. 

 The weathercock is in almost constant motion, swinging to and fro 

 through an arc often not less than a quadrant, and the consequence is, 

 that the pencil describes upon the cylinder, not a single line, but a 

 broad path of irregular form, made up of the transverse lines' which 

 the oscillation of the vane occasions. It might at first be supposed 

 that this oscillation arose from the momentum of the vane, and might 

 be remedied bv some contrivance whicli should cause the change of 

 direction of the wind to come into effect more slowly ; such for example 

 as the tail of a windmill. But the cause of this oscillation is in reality 

 almost entirely the constant shifting of the wind, as may be seen by 



