October i, 190S] 



.VA TURE 



543 



altitudes only — a mininture dumpy level, fitting- on to 

 the same tripod which carries a full-sized azimuth 

 compass, reading to half-degrees. 



-\ crschoyle's pocket transit, showing the s'.de arm for measuremenl 

 of altitudes. 



Azimuth and altitude are also provided for in the 

 so-called Verschoyle pocket transit. In this the hori- 

 zontal plane is provided for by adjusting a bubble in 

 ■a short spirit level. 



The altitude arrangement is on the side of an a/i- 



Observing an azimuth with a small theodolite. 



muth compass, the graduations of which are read 

 from the side by a right-angled prism, the graduations 

 being cut on a bevelled edge. 



Observations of magnetic azimuth and altitude can 

 also be made by more complicated instruments, such 

 ai. theodolites, miners' dials, &c., if, as is generally 

 the case, a magnetic needle is provided for 

 determining the magnetic north point. 



The chief point about the theodolite in all 

 its forms is that, whether provided with a 

 needle to give magnetic north or not, observa- 

 tions of sun and stars can be made so that the 

 true or astronomical north can be found, 

 ill the readings of altitude made by anv of these 

 instruments, where trees, houses, &c., top the horizon, 

 they should, of course, be neglected, and the elevation 

 of the ground level at that spot taken. Should the 

 top of the azimuth mark (stone, &.c.) show above the 

 actual horizon, its elevation should be recorded, as 

 V ell as that of the horizon. 



l\iilrumcn\s for ilctcniiiniiii; Aslroiwmical Aziniiilh 



mid Altitude. 



Both for the determination of astronomical or true 



azimuths directly, and for accurate observations of 



altitude, a theodolite is essential. It is true, as has 



N'O. 2031, VOL. 78] 



Fig. 18.— Eyepiece attachment to a theodolite to enable observation of the 

 sun, or a star at a high altitude such as Polaris, to be made. 



been stated, that a theodolite armed with a magnetic 

 needle can provide us with magnetic bearings, but if 

 th.^ best use is to be made of it the needle should be 

 disc.'irdcd altogether. 



.-\ theodolite is a very complicated instrument, and 

 really little can be learned by a perusal of a descrip- 

 tion, however long or detailed. The best way of 

 learning how to use it is to get a friend to give you 

 instructions while you, yourself, take each part out of 

 its box, set it up, and then proceed to make some 

 observations with it. 



In using- a theodolite the various alignments re- 

 quired are referred to some fixed point on the horizon, 

 or at all events some distance away, and the angles 

 determined ; the true azimuth of the fixed point is 

 then found bv observations of the sun or a star. 



