Disturbance on the 24!th of September 184-7. 34-7 



At this time I was called, and we began to observe the trans- 

 portable declinometer and bifilar. The last also went oiF the 

 scale. The lowest reading of the former was 692*5 at 1^ 0"^ 

 Gott., and the highest 1 126*0 at l^' 45"^: this gives a range of 

 3° 36'*7; but at a subsequent period (5^ 0"^ Gott.) a reading of 

 1177*2 was obtained, thus giving the enormous range of 

 4° 2'"3*. I did not take a reading of your compass; but 

 looking hastily at it, I perceived that during the great shock 

 it was ranging more than 3° 20' from its usual position. As 

 both bifilar scales were exceeded, we can only say that the 

 range of that element between 0^ and l"* Gott. exceeded 600 

 divisions, or 0*052 of its whole amount, on the testimony of 

 two instruments ; a fact which cannot, 1 think, but make it a 

 most interesting question, what is the nature of a force sub- 

 ject to such immense variations, and how can they occur with- 

 out affecting or being affected by the other physical agents in 

 the globe? This disturbance was attended by a great degree 

 of motion in the magnets, a peculiar mechanical agitation, 

 which they only exhibit on rare occasions ; it lasted, more or 

 less, down to 12^ Gott. As the results have not been made 

 up, I cannot state precisely the range of inclination, but per- 

 haps may do so before I close this. 



After some little trouble, I think we have got Dr. Robin- 

 son's Anemometer into beautiful working order. If the prin- 

 ciple on which the velocity is estimated is correct, as we must 

 feel confident it must be, I think it has a great superiority over 

 any other instrument of the kind yet invented. The facility 

 and precision with which the velocity is measured, and the 

 beautiful manner in which sudden changes are shown, together 

 with the large scale on which directions are marked, make it 

 a pleasure to use it, and make Osier's instrument look quite 

 clumsy beside it; it is a most elegant instrument, and will give 

 diurnal curves of velocity with a precision we have never 

 attained before. I found on careful examination that Osier's 

 anemometer, which has been up seven years, was much the 

 worse for wear, and not in a condition to give a satisfactory 

 comparison with the other ; we have therefore, with a good 

 deal of difficulty, taken it down. I have put it into the hands 

 of an engineer here, and he is to refit all the essential parts, 

 particularly the shoulder and collar of the vane, which were 

 worn, and made the vane unsteady : we shall then be able to 

 compare pressures and velocities. 



Believe me, my dear Colonel, 



Faithfully yours, 



J. H. Lefroy. 



• I think our greatest range before this was only 2° 23'; this occurred 

 last April. 



