Second button in linear layout not displaying

I have a layout for an Android activity in which the second button is not displaying:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="#659EC7"
android:weightSum="4"
>
<LinearLayout android:layout_height="0dp" android:layout_weight="1" android:layout_width="fill_开发者_如何学Cparent" android:id="@+id/linearLayout3">
    <TextView
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:text="Top Text"
    android:textSize="30dip"
    android:gravity="center"></TextView>
</LinearLayout>
<LinearLayout android:layout_height="0dp" android:layout_weight="3" android:layout_width="fill_parent" android:id="@+id/linearLayout7"  android:weightSum="4">
    <Button 
        android:id="@+id/Button1" 
        android:layout_width="fill_parent"
        android:layout_height="wrap_content" 
        android:text="Button1" 
        android:onClick="onButton1Click" />
    <Button 
        android:id="@+id/Button2" 
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="Button2"
        android:onClick="onButton2Click" 
         />
</LinearLayout>

Put android:orientation= "vertical" in the LinearLayout that is containing both the buttons