Mr Scouler on the Temperature of N. W. America. 251 



made upon the mirror, and the parts are so adjusted, that 

 when the wire AB is in contact with the upper limb of the sun 

 and the image of the bubble EF exactly between the marks 

 M and N, the axis of the telescope is in a true horizontal line. 



In making this observation, the eye sees by direct vision 

 the contact of the sun with the wire AB, and by oblique vi- 

 sion, the contact of the bubble with one or both of the marks 

 M, N. This double and simultaneous observation is difficult 

 to make ; but, independent of this difficulty, there is a pro- 

 perty of vision, in virtue of which an object seen obliquely 

 disappears as if it had been completely annihilated. 



On this and other grounds Mr Adams's eye-tube has al- 

 ways appeared to me susceptible of improvement. The first 

 idea of this kind which occurred to me is shown in Fig. 6, 

 where the field of view ACBD is contracted, and consists of a 

 perforation in the reflecting mirror. The parts are then ad- 

 justed, so that when the wire AB touches the sun, the bub- 

 ble AMBN is concentric with the field of view ACBD. This 

 approximation of the bubble to the observed limb of the sun 

 is an obvious advantage, but as it is liable to the objection 

 formerly stated against oblique vision, I thought of the me- 

 thod shown in Fig. 7. In this method I dispense entirely 

 with a metallic reflector, and I form the image of the bubble by 

 a plate of parallel glass PD, lying between the eye and the field 

 CD, and inclined 45° to the axis of the tube. By this means 

 the bubble may be brought in contact with the wire AB ; and 

 the parts are adjusted, so that the axis of the telescope is ho- 

 rizontal when the wire AB is in contact with the lower end of 

 the bubble, and the upper surface of the sun. The only ob- 

 jection to this construction is, that the glass plate PD reflects 

 little light ; but this may be completely remedied by placing 

 the darkening glass anterior to the field of view, or by throw- 

 ing an additional light upon the bubble of the level. 



Art. XVI.— On the Temperature of the North West Coast of 

 America. By Mr Scouler. Communicated by the Author. 



Every traveller who has visited the N. W. coast of America 





