C 5*3 ) 
Now to apply this (in a few words) to the Tt umpet, 
where the Notes are produced only by the different force 
of the breath; it is reafonable to imagine that the • 
Itrongeftblaftraifesthe found by breaking the Air with- 
in the Tube into the (horteft vibrations, but that no Mu- 
fical found will arife unlefs they are futed to fome ali- 
quot part, and fo by reduplication exa&ly meafure out 
the whole length of the Inftrument, as in Fig.C, for 
otherwife a remainder will caufe the fame inconvenience 
in this cafe, as in Fig. £). To which if we add that a Pipe, 
being (hortned according to the Proportions we even 
now difcours'd of in a String, raifes the found in the 
fame degrees, it renders the cafe of the Trumpet juft 
the fame with the Monochord. 
For a Corollary to this Difcourfe, we may obferve 
that the diftances of the Trumpet Notes afcending, con- 
tinually decreafed in proportion of i i £ t f.in infinitum^ 
For, 
Second /Note in thetfirft 7 \ i 7 ^ c t - tn „ 
The)third ^able 5 difrcrs)fecond>by>t > o{ . the Strln &> ' 
/fourth Sfrom the {third > ? f } 
An Account of the caufe of the Change of the Variati- 
on of the Magnetical Needle, with an Hypothefisof 
the StruBure of the Internal parts of the Earth : 
as it was propofedto the Royal Society in one of 
their late Meetings. 'By Edm. Halley. 
SOme years fince I publiflied in thefe Tranfa&ions, 
(Numb. 148.) a Theory-oi^he Variation of the 
Magnetical Compafs, wherein having colle&ed as many 
Obfervations as at that time I could procure, and having 
carefully 
